errors: a save that could not happen, and two panics on an ordinary click

A review of what this program does when the environment says no. The finding
that reframes it: there were almost NO panics on ordinary paths — the rule
already held — but there was a great deal of silence, and one case worse than
any panic.

SILENT DATA LOSS ON SAVE. `saveFile` marked the pane saved the moment it
QUEUED the effect, before any host had tried; `host_io.writeFd` returned void,
so a short or failed write was indistinguishable from a complete one; and
`writeFileBytes` returned true regardless. A save to a read-only file, or into
a directory removed under the pane, therefore cleared the tag's ` *` and posted
nothing — and `Del` makes no dirty check, so the next click threw the edits
away with the screen saying they were safe. On a full disk it was worse: the
file is already `O_TRUNC`'d when `write` fails, so the message row said `saved`
over a file that had just been emptied.

Now: `writeFd` reports, `writeFileBytes` returns WHY (`PermissionDenied`,
`NoSpaceLeft`, `ReadOnlyFilesystem`, …) including a failed `close`, which is
where write-back filesystems report at all; the core marks the pane saved
around `perform` rather than at emit, which is also where the bytes are read;
and a host that could not write calls `Pardes.saveFailed`, which puts the
reason on the message row and takes the clean mark back. That is a CALL and
not a return value because host.zig enforces, at comptime, that a `push_`
method reaching every host in a fan-out cannot have one answer — the first
attempt at this changed the signature and the compiler was right to refuse it.

TWO PANICS ON AN ORDINARY KEYSTROKE, in look.zig's number scans. `v = v * 10 +
d` over caller-supplied digits, reached from `parsePathLine` and the `@pN` scan
— which every Look, every right-click and every n/N motion runs on whatever
word is under the pointer. A hash in a log, a CSV column, any output shaped
`foo:99999999999999999999`, and the editor died with "integer overflow". Both
saturate now, the same way acmefs.zig's address parser already did; a saturated
line is refused by `file_pane.open`'s `line <= total` and a saturated pane id
by `focusPaneLine`'s `id < MAX_PANES`, so nothing addressable changes.

A BOOT FILE THAT WILL NOT OPEN joins the missing-name case in the `+Errors`
pane instead of taking the launch down: `pardes /root` resolves as a `.file`,
could not be read, and left `error: PermissionDenied` and a return trace.
`look.readFile` now says which errno it was, so the pane can say "permission
denied" rather than a word from the source code.

The tag-marker test drained no effects and passed anyway, which is exactly the
defect; it drains now.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_016Q4RATpafkwahrovHQLKRf

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