ADR-0022 stage 5/8: hint panel ambient typing assistance

ParseError::Invalid gains an `expected: Vec<String>` field —
the human-rendered names of the patterns chumsky was looking
for at the failure point (`\`create\``, `identifier`, etc.).
Empty for custom errors, which have no expected-set framing.
Populated by a new `describe_expected()` helper in parser.rs
that humanise() also delegates to (eliminates duplication).

`input_render::ambient_hint(input) -> Option<String>` returns
the hint-panel content per ADR-0022 §6:
  - empty input → None (caller falls back to panel.hint_empty);
  - Valid → t!("hint.ambient_complete") ("submit with Enter");
  - IncompleteAtEof → t!("hint.ambient_expected", expected = …)
    listing the parser's expected next tokens, oxford-joined;
  - DefiniteErrorAt → t!("hint.ambient_error_with_usage", …)
    composing the parse-error message with the matching
    parse.usage.* template if a known entry keyword was
    consumed, else the bare message.

Catalog gains the three hint.ambient_* keys + validator
declarations.

ui::render_hint_panel resolution order:
  1. explicit app.hint (modal contexts) wins;
  2. simple-mode + non-empty input → ambient_hint;
  3. fallback to panel.hint_empty.
Advanced mode (persistent + one-shot `:`) bypasses ambient
hinting per ADR-0022 §12.

Snapshot: highlighted_input_all_token_classes rebaselined
because the hint panel now displays an ambient hint instead
of the empty placeholder when input is non-empty.

Tests: 698 passing, 0 failing, 1 ignored (693 baseline →
+5 ambient_hint cases). Clippy clean.

Stage 6 introduces the IdentSlot taxonomy + parser audit so
identifier-typed slots can yield schema-aware completion
candidates in stage 8.

src/snapshots/rdbms_playground__ui__tests__highlighted_input_all_token_classes_dark.snap

@@ -22,7 +22,7 @@ expression: snapshot
 │                          ││insert into T values (1, 'hi', null) --all-rows $ │
 │                          │╰──────────────────────────────────────────────────╯
 │                          │╭ Hint ────────────────────────────────────────────╮
-│                          ││(no active hint)                                  │
+│                          ││after `insert into T values (1, 'hi', null)`,     │
 ╰──────────────────────────╯╰──────────────────────────────────────────────────╯
 Project: Term Planner                                                           
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