fix(web): an advertised key is never silent, a formula is visible, and +cat commits

improvise-rmvx, improvise-fdt6, improvise-gcvb — three faces of the same
thing: a browser offering something and then doing nothing.

rmvx. `the_command_names_that_still_need_the_host_are_these` was built with
`in_mode(AppMode::Normal)`, so it only ever asked about Normal-mode bindings:
`add-record-row` (RecordsNormal) and `delete-formula-at-cursor` (FormulaPanel)
were outside its reach entirely and could never have failed it. A named list
that silently scopes itself to one mode is a guard that looks broader than it
is. `every_advertised_command_answers_in_its_own_mode` crosses
`help::advertised` — command NAMES, so no hint string is tokenised — against
`resolve` in every mode, over a fixture furnished enough that an `Upstream` can
only mean an unhandled name. It found a seventh instance the ticket had not
predicted: `view-back` is advertised in RecordsNormal.

Six names now decline out loud, each with its own true subject, out of one
`declined` table; `stranding` returns the subject rather than a bool, so `:`
says the command line is unavailable instead of doing nothing. A declining step
takes its whole `Binding::Sequence` with it, which is what keeps `o` ("add a
record row, then edit it") from opening an editor on a row nobody created.

fdt6. `commit-buffer formula` already resolved; what did not exist was any way
to SEE the typing. `RenderModel::editing` is scoped to the two cell-editing
modes and reads the "edit" buffer by name, so FormulaEdit, CategoryAdd, ItemAdd
and the three prompts opened and swallowed every character. `RenderModel::
minibuffer` is `AppMode::minibuffer()` plus the buffer that config names, read
through `improvise_protocol::read_buffer` so gotcha 11 keeps its one home, and
`web/app.js` paints it.

gcvb. The records-mode early return was broader than its reason. A
`CellId::Create` commits the typed NAME as one existing `ModelCommand`, reads
no `record_col` and writes no cell, so the return narrows rather than
disappears. `+cat` must NOT advance: an inserted column renumbers every index
to its right and the advance was computed from the pre-commit layout, so a
client that advanced anyway would land one column off, silently. Both halves
proved by injection against `CommitAndAdvance` in `tests/lockstep.rs`.

Every new guard was proved by injecting the fault it names. The one worth
repeating: `web/tests/render_surface.test.js` went green over a page that
painted nothing, because the string `model.minibuffer` was inside a function
nothing called. The static check is a completeness check, never an execution
check — `harness/tests/keys.spec.js` caught it in Chromium on the first run.

1394 → 1397 Rust tests; harness 17 → 19 passed, 3 skipped; node 26.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

web/index.html

@@ -83,6 +83,13 @@
       <div id="grid" tabindex="0" aria-label="Grid"></div>
     </main>
 
+    <!--
+      The bottom bar. The TUI draws the minibuffer INSTEAD of the status line;
+      here it is a line of its own, because #status also carries the transport's
+      own failures and a repaint must not erase one (gotcha 48).
+    -->
+    <div id="minibuffer" role="status" hidden></div>
+
     <div id="help" role="dialog" aria-label="Help" hidden></div>
 
     <script type="module" src="./app.js"></script>