ADR-0024 Phase A: walker framework + app-lifecycle commands

Stand up the unified-grammar tree walker alongside the existing
chumsky parser and migrate the eleven app-lifecycle commands
(quit, help, rebuild, save / save as, new, load, export, import,
mode, messages) end-to-end. The router in parse_tokens consults
the walker first; non-migrated commands still fall through to
chumsky.

Scope:
- src/dsl/grammar/{mod,app}.rs: Node enum (13 kinds), Word /
  IdentSource / HintMode / HighlightClass / ValidationError /
  CommandNode types, REGISTRY of the eleven app commands.
- src/dsl/walker/{mod,driver,context,outcome,lex_helpers}.rs:
  scannerless byte-level walker, per-node-kind dispatch with
  Choice/Seq/Optional backtracking, WalkContext (Phase B-D
  schema fields stubbed), WalkOutcome with Match/Incomplete/
  Mismatch/ValidationFailed.
- src/dsl/parser.rs: try_walker_route() runs first in
  parse_tokens; bridge converts WalkOutcome to ParseError
  preserving catalog wording (mode.unknown / messages.unknown
  surface verbatim via friendly::translate). Legacy
  try_parse_app_path_command deleted; chumsky's bare-keyword
  app branches remain unreachable until Phase F sweep.

Walker design choices worth noting:
- mode <value> / messages <value> use Choice(Word, Word, Ident)
  so known keywords appear in the expected-set; the trailing
  Ident catch-all funnels unknown values into the friendly
  validator that always errors with the catalog wording.
- save / save as is one CommandNode (Optional(Word("as"))) -
  closes the round-5 "save Tab can't offer as" limitation
  structurally.
- Path-bearing UX shipped per ADR-0024: BarePath terminates at
  whitespace; paths with spaces use the (not-yet-wired) quoted
  form. Existing tests pass on the new shape.

Tests:
- 28 new walker-specific tests in dsl::walker::tests covering
  every app-lifecycle command, friendly-error wording for
  mode/messages unknown values, trailing-garbage detection,
  whitespace tolerance, and routing fall-through.
- Total: 805 passed, 0 failed, 1 ignored (was 777 / 1).
- cargo clippy --all-targets -- -D warnings clean.

src/dsl/mod.rs

@@ -11,6 +11,7 @@
 
 pub mod action;
 pub mod command;
+pub mod grammar;
 pub mod ident_slot;
 pub mod keyword;
 pub mod lexer;
@@ -19,6 +20,7 @@ pub mod shortid;
 pub mod types;
 pub mod usage;
 pub mod value;
+pub mod walker;
 
 pub use action::ReferentialAction;
 pub use command::{