ADR-0024 Phase F (full) step 2: usage via CommandNode.usage_ids

Migrates parse-error usage-block rendering from the legacy
`dsl::usage::matched_entry` (which scanned a `Vec<Token>` for the
first matched Keyword) to walker-side lookup driven by each
`CommandNode`'s `usage_ids` slice.

`CommandNode.usage_id: Option<&'static str>` becomes
`usage_ids: &'static [&'static str]`. Multi-form families
(`drop`, `add`, `show`) carry every variant — `drop` lists
table/column/relationship templates; `add` lists column /
relationship; `show` lists data / table. The single-shape
commands carry their single catalog key.

App-lifecycle CommandNodes had pointed at non-existent
`parse.usage.app.*` keys (never noticed because the field was
unused); they now point at the real catalog entries
(`parse.usage.quit`, `parse.usage.help`, …).

New helpers in `dsl::grammar`:
- `usage_keys_for_input(source) -> Option<(entry_word, usage_ids)>`
  resolves the first identifier-shape token to a CommandNode and
  returns its usage_ids list. Used by `app::render_usage_block`
  and `input_render::ambient_hint`.
- `entry_words_alphabetised() -> Vec<&'static str>` replaces
  `dsl::usage::entry_keywords_alphabetised`.

`dsl::usage` is deleted. The "available commands:" fallback in
`render_usage_block` now formats entry words as `` `<word>` ``
directly (matching the `parse.token.keyword.*` catalog renders);
the per-keyword catalog wrappers will collapse in the next step
(ADR-0024 §cleanup-pass §F).

`parse_command` and `parse_tokens` slim down:
- `parse_command(input)` no longer pre-lexes — the walker scans
  source bytes directly.
- `parse_tokens` (internal-only `pub` for "future I3/I4 work")
  is removed; its body folded into `parse_command`.
- `unknown_command_error` reads the walker registry directly.

Touched modules also drop their `crate::dsl::lexer::lex` and
`crate::dsl::usage` imports: `app.rs`, `input_render.rs`,
`completion.rs`.

Tests: 852 passing, 0 failing, 1 ignored (down from 860 because
the 8 `dsl::usage::tests::*` tests are gone with the module).

src/dsl/mod.rs

@@ -18,7 +18,6 @@ pub mod lexer;
 pub mod parser;
 pub mod shortid;
 pub mod types;
-pub mod usage;
 pub mod value;
 pub mod walker;