feat(protocol): one home for UI strings (improvise-x7sw, ktv1, k099)

Three tickets asked one question — where do UI strings live relative to a
serde wire enum — and answering it three times is the disease this unit exists
to cure. One answer, in `improvise-protocol/src/help.rs`:

**A UI string is keyed by a command name, never by a key, and never hangs off
the enum that crosses the wire.** `AppMode::label`/`hint_text` are gone;
`help::mode_label`/`help::hint_line` replace them, so the enum a client
deserializes carries no English at all (improvise-k099). **And wherever a
string names a key, the key is looked up rather than spelled**:
`Hint { keys, command, caption }` takes `keys` off `Keymaps::default_keymaps()`,
`command` is the semantic value a localized front end keys off, and `caption`
is the default English — the `AxisColor`/`axis_fg` shape.

- **improvise-x7sw**: `help::which_key` is the which-key popup's rows, keys and
  captions together. The client already holds the keymaps, which is the answer
  the ticket preferred, so `ViewState::transient_prefix` stays an address.
- **improvise-ktv1**: the help pages are data — `help::pages()` — with every
  key row derived. `src/ui/help.rs` keeps its ratatui styles and loses ~310
  lines of content; `web/app.js` paints rows it does not author, and
  `harness/tests/shell.spec.js` asserts that `?` really draws them.

**What this deletes is the measure of it.**
`every_hint_key_resolves_in_its_mode` and its 60-line hint tokenizer are gone:
they existed to hold a hand-written string honest against the keymap, and the
string is derived now, so the property is true by construction
(`doctrine(a_consolidation_that_keeps_its_compatibility_test_is_unfinished)`).
`Keymap::binding_hints` goes with them — `which_key` is the one walker, and the
binding dump reads it. What replaced the test checks the *other* direction, a
mode advertising a command nothing binds, which was never checkable before.

Two defects the derivation exposed, both fixed here: `key_label` dropped
modifiers, so Ctrl-arrows read as bare arrows, and `Char(' ')` printed a literal
space instead of `Space`.

**What is deliberately not derived**, and says so: the import wizard's keys
(its whole keymap is one `Any` binding into `HandleWizardKey`, which decides
per step inside the command — a real hole, left visible), `gg`/`yy`/`ZZ` and
the ex-commands (`HelpLine::Literal`), and the captions, which no test can
judge. `context/code/protocol.md` §8 has the argument.

Cost, stated: the hints changed shape. They are longer, they name arrow keys
beside `hjkl`, and two sub-lists on the Panels page lost their visual indent.
That is the price of the keys being true.

Evidence: 1321 tests pass, and the 7 Chromium shell tests pass against the
assembled site. Two faults were injected into the help overlay — dropping the
`paintHelp` call, and removing the `mode != Help` guard so it is never hidden —
and each fails the new spec.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

context/test-evidence/protocol.md

@@ -398,3 +398,41 @@ An expanded name the cache does not carry must not shift the flattening:
 `expanded_cats` is per-client and survives a document swap, so it **routinely**
 names categories that are gone. A flattening that indexed by expanded-name
 would go off the end here.
+
+### only_quit_advertises_nothing
+
+improvise-2hi, carried over from `only_quit_has_no_hint` when the hints moved
+out of `AppMode` (improvise-k099). The old `hint_text` had a `_ => ""` arm, so
+the modes without hints were invisible; `ExportPrompt` was blank too
+(improvise-1c9), and the status bar went empty at the one moment the user has
+to be told what to type. Only `Quit` may say nothing — it is never rendered.
+
+What it asserts now is one step stronger, because `hint_line` is derived: a
+mode goes quiet not by someone forgetting a string but by every command it
+advertises becoming unbound, which
+`every_advertised_hint_names_a_binding_that_exists` catches first.
+
+### every_advertised_hint_names_a_binding_that_exists
+
+The replacement for `every_hint_key_resolves_in_its_mode` (improvise-2hi),
+which is **deleted**, and the deletion is the point
+(`doctrine(a_consolidation_that_keeps_its_compatibility_test_is_unfinished)`).
+
+That test existed because `AppMode::hint_text` was a hand-written string
+promising what the keys do, with nothing tying it to the keymaps. It walked
+every `Key:verb` token in every hint through a 60-line tokenizer that had to
+know the hint dialect — named keys, `Ctrl-<c>`, `F/C/V` alternatives, runs like
+`hjkl`, the em-dash prose tail, CommandMode's ex-commands — and panicked on a
+token it could not classify, because a token it skipped was exactly the one
+that would go unchecked.
+
+None of that is needed now. A hint names a **command**, and its keys are looked
+up in `Keymaps::default_keymaps()`, so "the hint names a key that resolves" is
+true by construction and cannot be otherwise. What remains checkable, and is
+what this test checks, is the other direction: a mode advertising a command
+that nothing binds. That was never checkable before.
+
+Note what neither version reaches, and gotcha 14 stated for the old one: that
+the *caption* is accurate. `i:edit` where `i` enters records mode still passes.
+The caption is English beside a command name, and only a reader can judge it.
+