Measure the explorer, and cap edit mode at what it can carry

Three numbers, taken on the emulator through the app's own render report
and written into EXPLORER.md; the fixture tree the sandbox now builds is
what they were taken against.

The viewer's scan was on the main thread. Decision 8 said off it, and the
first version did it in a `remember` inside the composition, which is not
that -- 460ms of frozen screen on a 1 MiB file, long enough that the
accessibility tree cannot be read, which is exactly what "the app has
stopped" looks like from outside. It runs on Dispatchers.Default now, with
a spinner where the file will be. Reading a megabyte is otherwise fine:
the viewer is a row per line, and it opens and scrolls 28,660 of them.

Edit mode needed a cap, and not the one the plan expected. The cost that
matters is not the highlighting -- 40ms a keystroke at 128 kB, which is
survivable -- it is Compose laying out one enormous text in the field:
2,027ms per frame at 128 kB, with typed characters dropped, and no
response at all at 1 MiB. Switching highlighting off would have saved
nothing, since every arrangement of a single text field pays it. So
EDIT_LIMIT is 32 kB, the largest size actually measured as usable, and
above it the pencil is disabled with the reason in words beside it: a
disabled control teaches what the thing can do but cannot say why it is
off, and a reader who cannot edit a file they can plainly read would
otherwise conclude the app is broken.

`FileLines.of` is timed like everything else here, so the figure lands in
the render report rather than needing a harness to ask for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -149,6 +149,13 @@ repo is in PLAN.md's "Backend layout" section.
   `./ui-sandbox.sh api /sessions/<id>/cwd -X POST -H 'content-type: application/json' -d '{"cwd":"~/files"}'`.
   The 409 is produced by editing the file on the machine (`printf … > file`)
   between pressing the pencil and pressing save.
+  **Reading is cheap and editing is not**, and the sizes are measured
+  rather than guessed -- see EXPLORER.md's "What the measurements said".
+  The viewer handles a 1 MiB, 28,000-line file because it draws one row per
+  line; the editor is one `BasicTextField`, which costs two seconds a frame
+  at 128 kB and stops the app at 1 MiB, so `EDIT_LIMIT` caps it at 32 kB
+  with the reason said on screen. If you make the editor faster, that
+  number is what to move.
 - `.dev-updater.ron` — what Dev Updater is asked to do with this checkout:
   the server (built in `server/`, run as `service: Managed(...)`) and the
   APK (built in `app/`), built in parallel. The project it serves is the