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/mod.rs

@@ -229,12 +229,46 @@ pub struct CommandNode {
     pub ast_builder: fn(&MatchedPath) -> Result<Command, ValidationError>,
     #[allow(dead_code)]
     pub help_id: Option<&'static str>,
-    #[allow(dead_code)]
-    pub usage_id: Option<&'static str>,
+    /// Catalog keys under `parse.usage.*` to render in the
+    /// "usage:" block when a parse error fires for this command
+    /// (ADR-0021 §1, ADR-0024 §architecture). Multi-form families
+    /// like `drop` (drop table / drop column / drop relationship)
+    /// carry every variant so the user sees the full family on a
+    /// generic-entry-word failure.
+    pub usage_ids: &'static [&'static str],
     #[allow(dead_code)]
     pub hint_mode: Option<HintMode>,
 }
 
+/// Look up the usage catalog keys for the entry word at the start
+/// of `source`.
+///
+/// Case-insensitive, whitespace-tolerant. Replaces
+/// `dsl::usage::matched_entry` — the walker is the single source
+/// of truth for which command a given input belongs to.
+///
+/// Returns the canonical (primary-form) entry literal and the
+/// `usage_ids` list, or `None` if no entry word matches.
+#[must_use]
+pub fn usage_keys_for_input(source: &str) -> Option<(&'static str, &'static [&'static str])> {
+    use crate::dsl::walker::lex_helpers::{consume_ident, skip_whitespace};
+    let start = skip_whitespace(source, 0);
+    let (kw_start, kw_end) = consume_ident(source, start)?;
+    let word = &source[kw_start..kw_end];
+    let (_, node) = command_for_entry_word(word)?;
+    Some((node.entry.primary, node.usage_ids))
+}
+
+/// Every command-entry word in the registry, sorted alphabetically
+/// by primary literal. Replaces `dsl::usage::entry_keywords_alphabetised`
+/// which read the same data through the legacy `usage::REGISTRY`.
+#[must_use]
+pub fn entry_words_alphabetised() -> Vec<&'static str> {
+    let mut words: Vec<&'static str> = REGISTRY.iter().map(|c| c.entry.primary).collect();
+    words.sort_unstable();
+    words
+}
+
 /// The active grammar registry. Phase A: the eleven app-lifecycle
 /// commands. Migrated commands route through this; everything
 /// else falls through to the chumsky path in `dsl::parser`.