Look and Exec are builtins like any other

Their only special feature is now the keys they are assigned. `Look main.zig`
typed in a tag is the same look a right click is; config.look_cmd/exec_cmd
point the two buttons and Enter/Tab at them, so retargeting Enter to Grep is
one line. actOnSelection — a hand-written cascade with the builtin dispatch
nested inside it — is gone, and the `button` parameter it threaded through five
KEYBOARD call sites went with it.

New syntax, spelled once in config.zig: @`ls -la` names a command to run
rather than a file to open. Word expansion takes the quoted run whole, the way
acme does for its own </|/> words, so a click inside one does not hand Look the
fragment `ls`.

And it nests: @`Look .` unwraps, re-enters the dispatcher, and looks at the
directory. Guarded at depth 8, which is unreachable today (every re-entry
strips a word or a delimiter pair, so the string strictly shrinks) and exists
for a future syntax that does not shrink.

leader_path is now optional: a builtin may have no SPC path when its shortcut
is a key and a mouse button. New golden cmdword; the other 59 unmoved.

2 files changed, 295 insertions, 0 deletions

diff --git a/test/snapshots/cmdword.golden b/test/snapshots/cmdword.golden
new file mode 100644
index 00000000..03e272be
--- /dev/null
+++ b/test/snapshots/cmdword.golden
@@ -0,0 +1,217 @@
+== snap opened grid=120x30 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
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+== snap atexec grid=120x30 cursor=7,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
+|
+|                                                              ZZE
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+== snap atlook grid=120x30 cursor=64,8
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
+|
+|                                                              ZZE
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+|                                                              ZZL
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+== snap lookword grid=120x30 cursor=7,17
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
+|
+|                                                              ZZE
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+|                                                              ZZL
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+|  NOR /tmp/pardes-snap/cmdword/cwd/p.txt Save Del
+|     1 just text
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+== snap nested grid=120x30 cursor=64,10
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          NOR /tmp/pardes-snap/cmdword/cwd Del
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
+|
+|                                                              ZZE
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+|                                                              ZZL
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+|                                                              cmds.txt  p.txt  zzmark.txt
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+|  NOR /tmp/pardes-snap/cmdword/cwd/p.txt Save Del
+|     1 just text
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+== snap execword grid=120x30 cursor=102,1
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          INS /tmp/pardes-snap/cmdword/cwd Del Exec
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
+|
+|                                                              ZZE
+|
+|                                                              ZZL
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+|                                                              cmds.txt  p.txt  zzmark.txt
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+|  NOR /tmp/pardes-snap/cmdword/cwd/p.txt Save Del
+|     1 just text
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+== snap tagcmd grid=120x30 cursor=116,1
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/cmdword/cwd/cmds.txt Save Del          INS /tmp/pardes-snap/cmdword/cwd Del Exec @`echo ZZ''T`
+|     1 @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+|                                                              cmds.txt  p.txt  zzmark.txt
+|
+|                                                              ZZE
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+|                                                              ZZL
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+|                                                              cmds.txt  p.txt  zzmark.txt
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+|                                                              ZZT
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+|  NOR /tmp/pardes-snap/cmdword/cwd/p.txt Save Del
+|     1 just text
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diff --git a/test/snapshots/cmdword.snap b/test/snapshots/cmdword.snap
new file mode 100644
index 00000000..f4d4fed4
--- /dev/null
+++ b/test/snapshots/cmdword.snap
@@ -0,0 +1,78 @@
+# `@`...`` — a word that names a COMMAND instead of a file to open. The
+# delimiters are config.cmd_open/cmd_close and nothing else in the editor knows
+# how one is spelled.
+#
+# A backtick is not a file char, so the ordinary word expansion would stop dead
+# inside one and hand a look the fragment `echo`. config.wordBounds takes the
+# whole run instead, which is what makes a single CLICK the way you invoke one
+# (acme does the same for its own `<`/`|`/`>` command words).
+#
+# EXECUTE runs it and leaves you where you were; LOOK runs it and lands you in
+# the shell that answered — looking at a thing means being shown it, and a
+# directory's look has always been exactly that (a shell there, ls'd).
+#
+# And Look and Exec are ORDINARY BUILTINS now, so their names are executable
+# words like Save or Grep: `Look p.txt` run off the page is the same look a
+# right click on `p.txt` is. Which is also what makes `@`Look .`` nest — Exec
+# unwraps it and the contents go straight back through the one dispatcher.
+file cmds.txt @`echo ZZ''E` @`echo ZZ''L` Look p.txt @`Look .`
+file p.txt just text
+file zzmark.txt x
+start 30 120 cmds.txt
+wait 8000 Kill Newcol
+wait 8000 cmds.txt
+stable 700 20000
+snap opened
+# a single middle CLICK inside the run (col 11 is inside `echo`) expands to the
+# whole `@`echo ZZ''E`` and executes it in a shell; focus stays on the file
+press middle 11 3
+release middle 11 3
+wait 10000 ZZE
+stable 700 15000
+snap atexec
+# LOOK on the second one (col 24, inside its `echo`) runs it too AND focuses
+# the shell that answered — that is the whole difference between the two verbs
+press right 24 3
+release right 24 3
+wait 10000 ZZL
+stable 700 15000
+snap atlook
+# `Look p.txt` middle-dragged: the builtin by NAME, with an argument
+press middle 36 3
+drag middle 45 3
+release middle 45 3
+wait 10000 just text
+stable 700 15000
+snap lookword
+# nesting: `@`Look .`` — Exec unwraps it, `Look .` is parsed as the ordinary
+# Look builtin with an argument, and Look opens the directory (an existing
+# shell there, ls'd — zzmark.txt is only ever in that listing)
+press middle 49 3
+release middle 49 3
+wait 10000 zzmark.txt
+stable 700 15000
+snap nested
+# the same syntax typed into a TAG — the other place the expansion has to know
+# about backticks (tagChordText). The look above left the shell focused, so `:`
+# opens ITS tag; `i` then End walks to the end of the tail, and the chord runs
+# the word under the cursor exactly as the body's does.
+key :
+key i
+key end
+key space
+text Exec
+key left
+key tab
+stable 700 8000
+# ...except that word is `Exec` ITSELF, with nothing to execute. This is the
+# case that must not hang a re-entrant dispatcher: it runs out of argument
+# immediately and does nothing at all.
+snap execword
+key end
+key space
+text @`echo ZZ''T`
+key left
+key tab
+wait 10000 ZZT
+stable 700 15000
+snap tagcmd