ADR-0024 Phase B: DDL commands without value literals

Migrate the five DDL commands at four entry words: drop (drop
table / drop column / drop relationship), add (add column /
add 1:n relationship), rename (rename column), change (change
column). The walker route now owns these end-to-end; chumsky
declarations remain unreachable for these inputs but stay
until Phase F.

Walker extensions:
- New node kinds: NumberLit (with optional content validator)
  and Literal(&str) (verbatim byte sequence with word-boundary
  lookahead — used for the `1` in `add 1:n …` so it surfaces
  as `\`1\`` in the expected-set, matching the existing
  parse_error_pedagogy contract).
- Flag (--name) terminal — Phase A stubbed; now wired to the
  walker driver with consume_flag() in lex_helpers.
- Repeated combinator with optional separator and `min` floor.
  Used by referential clauses (0..2 `on <delete|update>` runs)
  and change-column flags (0..N --force-conversion /
  --dont-convert; AST builder enforces mutual exclusion).
- Optional now propagates its inner's expectations as a
  `skipped` field on the Matched result. Seq accumulates these
  across children so the next failure's expected-set surfaces
  the full union — closes the keyword-completion regression
  (`add column ` must offer `to`, `table`, plus the table-name
  identifier slot).
- Expectation::Ident gained a `source: IdentSource` field; the
  parser-side bridge maps Tables/Columns/Relationships/Types
  to the IdentSlot::expected_label strings ("table name",
  "column name", …) so the existing completion engine's
  schema-cache lookup still resolves.
- Walker error wording now includes "after `<consumed>`,
  expected …" framing — matches the chumsky-side test
  contract for structural errors mid-shape.
- AST-builder validation errors now propagate as
  WalkOutcome::ValidationFailed (not the generic "AST builder
  failed" fallback), so `change column … --force-conversion
  --dont-convert` and repeated `on delete` clauses surface
  their friendly catalog wording verbatim.

Grammar additions:
- src/dsl/grammar/shared.rs: type-name validator (TYPE_VALIDATOR
  uses Type::from_str via parse.custom.unknown_type catalog),
  qualified_column sub-grammar, referential action keyword
  (`cascade`/`restrict`/`set null`/`no action`), repeated
  on-clauses.
- src/dsl/grammar/ddl.rs: drop/add/rename/change CommandNodes
  with inline shapes (per-use-site `role` annotations let the
  AST builder discriminate parent vs child columns, etc.).
  The four entry words each have one CommandNode whose `shape`
  is a Choice across sub-forms.

Tests:
- 14 new walker-specific tests covering all DDL forms (bare
  drop table, drop column with optional connectives, drop
  relationship by name and by endpoints, add column with type
  validator, rename column, change column with each flag form
  + mutual-exclusion check, add 1:n relationship minimal /
  full, repeated-clause-twice rejection).
- Total: 819 passed, 0 failed, 1 ignored (was 805 / 1).
- cargo clippy --all-targets -- -D warnings clean.

src/dsl/walker/outcome.rs

@@ -11,7 +11,7 @@
 //! tests; completion + highlighting still flow through the
 //! chumsky path until Phase D / F.
 
-use crate::dsl::grammar::{HighlightClass, ValidationError};
+use crate::dsl::grammar::{HighlightClass, IdentSource, ValidationError};
 
 /// How far into the input the walker should consume.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -31,10 +31,20 @@ pub enum WalkBound {
 /// only what the router needs to render a parse error.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum Expectation {
-    /// The walker expected one of these literal keywords.
+    /// The walker expected this literal keyword.
     Word(&'static str),
-    /// The walker expected an identifier of the given role.
-    Ident { role: &'static str },
+    /// The walker expected this verbatim literal byte sequence
+    /// (used today for the `1` in `add 1:n …`).
+    Literal(&'static str),
+    /// The walker expected an identifier slot. `source` drives
+    /// the user-facing expected-label rendering ("table name",
+    /// "column name", …) so the existing completion engine's
+    /// `IdentSlot::from_expected_label` round-trip still works.
+    /// `role` is the walker-internal slot tag.
+    Ident {
+        role: &'static str,
+        source: IdentSource,
+    },
     /// The walker expected this exact punctuation character.
     Punct(char),
     /// The walker expected a number literal.