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/input_render.rs

@@ -25,6 +25,7 @@
 use ratatui::style::{Modifier, Style};
 
 use crate::dsl::lexer::lex;
+use crate::dsl::walker;
 use crate::dsl::{ParseError, parse_command};
 use crate::theme::Theme;
 
@@ -300,13 +301,16 @@ pub fn lex_to_runs(input: &str, theme: &Theme) -> Vec<StyledRun> {
 }
 
 fn base_runs(input: &str, theme: &Theme) -> Vec<StyledRun> {
-    let tokens = lex(input);
-    let mut runs = Vec::with_capacity(tokens.len() * 2);
+    // Walker-driven highlighting (ADR-0024 §architecture, Phase F).
+    // `walker::highlight_runs` returns per-byte classes for every
+    // token shape in the source; whitespace gaps are not represented
+    // and we fill them with the default foreground colour below.
+    let classes = walker::highlight_runs(input);
+    let mut runs = Vec::with_capacity(classes.len() * 2);
     let mut pos = 0;
-    for tok in tokens {
-        let (start, end) = tok.span;
+    for class in classes {
+        let (start, end) = (class.start, class.end);
         if pos < start {
-            // Whitespace gap before this token.
             runs.push(StyledRun {
                 byte_range: (pos, start),
                 style: Style::default().fg(theme.fg),
@@ -314,7 +318,7 @@ fn base_runs(input: &str, theme: &Theme) -> Vec<StyledRun> {
         }
         runs.push(StyledRun {
             byte_range: (start, end),
-            style: Style::default().fg(theme.token_color(&tok.kind)),
+            style: Style::default().fg(theme.highlight_class_color(class.class)),
         });
         pos = end;
     }