walker: 3a — category-grouped mode-aware dispatch (ADR-0033 Amendment 1)

Replaces ADR-0033 §2's original Node::Guard + Choice(SQL,DSL) mechanism,
which was found during 3a to be unworkable: any guard-in-Choice approach
forces a walk_choice change (walk_choice falls through only on NoMatch, so
simple-mode valid-DSL would wrongly surface "this is SQL"), and walk_seq
treats a NoMatch past idx 0 as a hard Failed, breaking advanced-mode DSL
fall-through.

Mechanism (Amendment 1): each REGISTRY entry is tagged
CommandCategory::{Simple, Advanced}, generalising the whole-command
is_advanced_only gate. walk() becomes a thin dispatcher over decide()
(mode-aware candidate selection: simple commits the DSL node or emits the
"this is SQL" hint; advanced tries SQL first, DSL as a full-line fallback)
and an extracted walk_one_command(); speculative match-testing runs on a
scratch WalkContext so the caller's context is only touched by the
committed walk. No Node::Guard, no walk_choice/walk_seq change.

6 dispatch smoke tests on a shared-entry-word smoke registry; 1446 baseline
green; clippy clean.

src/app.rs

@@ -1792,7 +1792,7 @@ impl App {
         // "DSL data commands" sub-header at the first command
         // whose help_id leaves the `app.` namespace.
         let mut dsl_header_done = false;
-        for command in REGISTRY {
+        for (command, _category) in REGISTRY {
             let Some(help_id) = command.help_id else {
                 continue;
             };