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/grammar/ddl.rs

@@ -549,7 +549,11 @@ pub static DROP: CommandNode = CommandNode {
     shape: DROP_SHAPE,
     ast_builder: build_drop,
     help_id: Some("ddl.drop"),
-    usage_id: Some("parse.usage.drop"),
+    usage_ids: &[
+        "parse.usage.drop_table",
+        "parse.usage.drop_column",
+        "parse.usage.drop_relationship",
+    ],
     hint_mode: None,
 };
 
@@ -558,7 +562,7 @@ pub static ADD: CommandNode = CommandNode {
     shape: ADD_SHAPE,
     ast_builder: build_add,
     help_id: Some("ddl.add"),
-    usage_id: Some("parse.usage.add"),
+    usage_ids: &["parse.usage.add_column", "parse.usage.add_relationship"],
     hint_mode: None,
 };
 
@@ -567,7 +571,7 @@ pub static RENAME: CommandNode = CommandNode {
     shape: RENAME_COLUMN,
     ast_builder: build_rename_column,
     help_id: Some("ddl.rename"),
-    usage_id: Some("parse.usage.rename_column"),
+    usage_ids: &["parse.usage.rename_column"],
     hint_mode: None,
 };
 
@@ -576,7 +580,7 @@ pub static CHANGE: CommandNode = CommandNode {
     shape: CHANGE_COLUMN,
     ast_builder: build_change_column,
     help_id: Some("ddl.change"),
-    usage_id: Some("parse.usage.change_column"),
+    usage_ids: &["parse.usage.change_column"],
     hint_mode: None,
 };
 
@@ -698,6 +702,6 @@ pub static CREATE: CommandNode = CommandNode {
     shape: CREATE_TABLE,
     ast_builder: build_create_table,
     help_id: Some("ddl.create"),
-    usage_id: Some("parse.usage.create_table"),
+    usage_ids: &["parse.usage.create_table"],
     hint_mode: None,
 };