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/theme.rs

@@ -18,6 +18,7 @@
 
 use ratatui::style::Color;
 
+use crate::dsl::grammar::HighlightClass;
 use crate::dsl::lexer::TokenKind;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -121,6 +122,25 @@ impl Theme {
             TokenKind::Error(_) => self.tok_error,
         }
     }
+
+    /// Map a walker `HighlightClass` to its display colour
+    /// (ADR-0024 §architecture, Phase F). This is the walker-side
+    /// equivalent of `token_color` — the renderer consumes
+    /// `walker::highlight_runs` output, which produces
+    /// `HighlightClass` per byte range, and looks up colours
+    /// through this method.
+    #[must_use]
+    pub const fn highlight_class_color(&self, class: HighlightClass) -> Color {
+        match class {
+            HighlightClass::Keyword => self.tok_keyword,
+            HighlightClass::Identifier => self.tok_identifier,
+            HighlightClass::Number => self.tok_number,
+            HighlightClass::String => self.tok_string,
+            HighlightClass::Punct => self.tok_punct,
+            HighlightClass::Flag => self.tok_flag,
+            HighlightClass::Error => self.tok_error,
+        }
+    }
 }
 
 impl Default for Theme {