ADR-0024 Phase F (full) step 1: walker-driven highlighting

Replaces the lex()-driven `base_runs` span builder in
`input_render.rs` with `walker::highlight_runs`. The new
walker-side `dsl::walker::highlight` module returns per-byte
`HighlightClass` assignments for every token shape in the source:

- For commands the walker engages on, `WalkResult::per_byte_class`
  is the authoritative source (keyword / identifier / number /
  string / punct / flag).
- Trailing junk past a partial match — and inputs the walker
  doesn't engage on at all (no registered entry word) — fall
  through to a byte-shape scanner over `lex_helpers` so unknown
  command words, stray punctuation, and unterminated strings
  still highlight sensibly.

`Theme::highlight_class_color` is the walker-side analogue of
`token_color(&TokenKind)`; the renderer reads `walker::highlight_runs`
output and looks up colours through it. `token_color` and the
`lex()` pre-pass remain in place for now — the lexer module is
still consumed by usage rendering and completion until the
remaining Phase F steps land.

`HighlightClass`'s and `WalkResult::per_byte_class`'s
`#[allow(dead_code)]` annotations come off — they're now part of
the production highlight path.

Tests:
- 16 new tests under `dsl::walker::highlight` cover end-to-end
  walks, byte-shape fallbacks (unknown commands, bare flags,
  numbers, punctuation), UTF-8 codepoint advance, and trailing-
  token handling after partial walks.
- Existing `input_render` tests pass unchanged.
- 860 total tests passing (727 lib + 133 integration), 1 ignored.

Clippy clean with `nursery` lints + `-D warnings`.

src/dsl/walker/mod.rs

@@ -14,6 +14,7 @@
 
 pub mod context;
 pub mod driver;
+pub mod highlight;
 pub mod lex_helpers;
 pub mod outcome;
 
@@ -27,6 +28,7 @@ use crate::dsl::walker::outcome::{
 };
 
 pub use context::ColumnInfo;
+pub use highlight::highlight_runs;
 
 /// Public walk entry. `bound` is `EndOfInput` for parse;
 /// `Position(cursor)` for completion / hint (Phase A: not yet