ADR-0024 Phase D: data commands at chumsky parity

Migrate the four data commands at four entry words: show
(show data / show table), insert, update, delete. Walker now
owns the entire command set introduced through ADR-0014.

Scope deviation from ADR-0024: full schema-aware value typing
via DynamicSubgrammar(column_value_list) is deferred. The
walker accepts any value at any position — matching the
existing chumsky parser's behaviour, where per-column type
checks happen at bind time. The DynamicSubgrammar Node
variant and WalkContext schema fields stay declared so the
infrastructure is in place when the schema cache plumbs
through parse_command (a future refinement). All existing
tests pass on the new shape.

Walker extensions:
- StringLit terminal — wired to the consume_string_literal
  helper that mirrors the legacy lexer's `''` escape handling.
  MatchedItem text carries the unescaped payload; span covers
  the surrounding quotes.
- Bridge: Incomplete error wording now appends `, found end
  of input` (matching the chumsky-side structural error
  contract that `structural_error_for_show_data_without_arg`
  asserts on).

Grammar:
- src/dsl/grammar/data.rs: SHOW (Choice of show_data /
  show_table), INSERT (three forms folded into a single shape
  via a Choice ordered to disambiguate Form B's `values`
  keyword from Forms A/C's `(`-prefixed content; the inner
  paren list is a Choice(VALUE_LITERAL, Ident{Columns}) with
  VALUE_LITERAL ordered first so `true`/`false`/`null` match
  their Word branch rather than the broader identifier catch-
  all), UPDATE (assignments + filter), DELETE (filter).
- VALUE_LITERAL = Choice(Word("null"), Word("true"),
  Word("false"), NumberLit, StringLit) — matches the chumsky
  `value_literal()`.
- WHERE_CLAUSE / FILTER_CLAUSE shared between update and
  delete.
- AST builders walk MatchedPath items in order, using role
  tags (`update_set_column`, `filter_column`,
  `insert_first_item`) to discriminate column references
  belonging to different shapes within the same command.

Tests:
- 13 new walker-specific tests covering all data forms:
  show data / show table, insert with each of three forms,
  insert with negative numbers, update with single + multiple
  assignments + where, update with --all-rows, delete with
  where, delete with --all-rows, update/delete without filter
  errors, replay still routes via chumsky.
- Total: 838 passed, 0 failed, 1 ignored (was 825 / 1).
- cargo clippy --all-targets -- -D warnings clean.

src/dsl/grammar/mod.rs

@@ -23,6 +23,7 @@
 //! value slots in Phase D.
 
 pub mod app;
+pub mod data;
 pub mod ddl;
 pub mod shared;
 
@@ -254,6 +255,10 @@ pub static REGISTRY: &[&CommandNode] = &[
     &ddl::RENAME,
     &ddl::CHANGE,
     &ddl::CREATE,
+    &data::SHOW,
+    &data::INSERT,
+    &data::UPDATE,
+    &data::DELETE,
 ];
 
 /// Look up a `CommandNode` by entry word, case-insensitively.