File the tag code into tagline.zig and draw tags beside bodies

With tags reduced to Texts, what is left of them is the computed prefix, the
default and saved tails, entering and leaving a tag and the headers: that goes
to tagline.zig, as acme keeps the tag half of a window in wind.c. Tag and
header drawing moves next to body drawing in body_layer.zig, and the tag hit
helpers go to tag_layer.zig with the Hit they read, where sameCell now also
tells the lines of a taller tag apart. The docs describe the tag as a Text.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 23 insertions, 21 deletions

diff --git a/docs/design.typ b/docs/design.typ
index 82bafb49..8e5f7e6e 100644
--- a/docs/design.typ
+++ b/docs/design.typ
@@ -530,9 +530,9 @@ Each pane kind keeps its storage and operations together in its own file
 image pane in `image.zig`), reached through `panes.zig` beside `Pane`.
 `layout.zig` owns placement and presentation state. `pardes.zig` handles input
 and cross-pane state directly, without a pane vtable; the rest of the editor
-is split by thing as acme is: `edit.zig`, `normal.zig`, `look.zig`, `exec.zig`,
-`mouse.zig`, `Messages.zig`, `Pipe.zig`, `colors.zig`, `surface.zig`,
-`body_layer.zig` and `dump.zig`. Integration fixtures live
+is split by thing as acme is: `Text.zig`, `edit.zig`, `normal.zig`, `tagline.zig`,
+`look.zig`, `exec.zig`, `mouse.zig`, `Messages.zig`, `Pipe.zig`, `colors.zig`,
+`surface.zig`, `body_layer.zig`, `tag_layer.zig` and `dump.zig`. Integration fixtures live
 in `test/panes.zig`, `test/output.zig`, and `test/pdf.zig`.
 
 = State
@@ -770,33 +770,35 @@ pair per theme, and the three per-button tints and the dimmed extra cursors are
 mixed off it, so what stays fixed is the distinction between buttons and not the
 colours.
 
-`Drag` is a `union(enum)` with six arms (`pardes.Drag`): `none`,
-`border_v`, `border_h`, `move`, `tag`, `select`. `border_v` carries an optional
+`Drag` is a `union(enum)` with six arms (`mouse.Drag`): `none`,
+`border_v`, `border_h`, `move`, `column_move`, `select`. `border_v` carries an optional
 `corner`: when the press lands on a cell that is both a column's vertical border
 and one of the two adjoining columns' own horizontal borders, the one drag moves
 *both* boundaries — never three, and when both columns happen to be split at the
 grabbed row the left one wins, so the gesture that existed before is bit-for-bit
 unchanged.
 
-== The tag is a command line
+== The tag is a text
 
-A tag is one line: a live prefix (mode indicator, cwd or path) plus an editable
-tail, and the tail gets the full modal editor. The coordinate space is UTF-8 byte
-offsets into the *whole* rendered tag, prefix ++ tail, always on grapheme
-boundaries (`Pane.tag_col`). The prefix is live chrome, so it is
-selectable, yankable and executable but READ-ONLY: every edit op measures from
-`edit0` — the first editable byte — and does nothing left of it.
+A tag is a `Text` (`Text.zig`), as a body is: acme's `Text`, one per
+`what` — the body, the pane's tag, the prompt line, a column's tag and the
+workspace tag. Each holds its own cursor, selections, mode and undo, so tags
+are edited by the same `edit.zig` and `normal.zig` code as bodies, and the one
+key they do not share is `:`, which moves between a pane's body and its tag.
+A tag holds any number of lines; a pane's tag takes a row per line up to
+`MAX_TAG_ROWS`, each drawn as a tag layer of its own.
 
-`tag_tail` is a fixed `[max_tag_tail]u8`, and the bound IS the storage
-(`limits.max_tag_tail`): every writer — `appendTag`, `tagInsert`,
-`restoreDumpTail`, the 9P `tag` file — refuses input that does not fit
-rather than truncating it. The schema limit and the buffer therefore can never
-disagree, which is what lets a dump reader reject data before copying it into a
-pane.
+A pane's tag shows a live prefix (path, dirty marker, PDF page) before the text
+it owns, and that prefix is computed at every read, never stored
+(`tagline.tagPrefix`). The Text's coordinates are offsets into its own text
+alone, so a rename or a save never moves its cursor. The one translation
+point is the whole line as shown: render, the mouse, Look, Exec and the 9P
+`tag` file see prefix ++ text, while keyboard motions and edits see only the
+text. Editing the path is a separate draft (`Pane.prompt = .name`) committed
+by Enter, since a buffer's name is not text it owns.
 
-A tag edit is always insert mode, so it hijacks the body's mode; `tag_mode`
-remembers what it hijacked and terminals restore it on exit, which is why
-clicking a tag never changes a pane's mode.
+The tag's text is bounded by `limits.max_tag_tail`: the 9P `tag` file and a
+dump reader refuse input that does not fit rather than truncating it.
 
 = Diffing and presenting a frame