feat(commandline): autocomplete suggestions + inventory-based registry

Type-as-you-go suggestion popup above the input that filters known
command names by substring (so `LEADER` surfaces both LEADER and
MLEADER). Up / Down navigates the list, Enter dispatches the
highlighted entry, clicking a row dispatches directly. Esc on a
non-empty input clears the buffer instead of escalating to
command-cancel.

The name corpus comes from `inventory::submit!` blocks placed next
to each `CadCommand` impl rather than a hand-maintained static list.
Adding a new interactive command means dropping one `inventory::submit!`
line in its own module file — the autocomplete picks it up at the
next compile. 98 submissions seeded across 60 module files;
`command::all_registered_command_names()` flattens them at runtime.

One-shot dispatch arms in `app/commands.rs` (NEW, OPEN, SAVE, UNDO,
…) don't have a `CadCommand` impl and are therefore absent from
autocomplete by design.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/command/mod.rs

@@ -317,3 +317,29 @@ pub trait CadCommand: Send {
     /// Default: no-op.
     fn inject_picked_entity(&mut self, _entity: acadrust::EntityType) {}
 }
+
+// ── Autocomplete registry ─────────────────────────────────────────────────
+//
+// Every `impl CadCommand for Foo` module submits the names it answers to
+// at compile time via `inventory::submit!`. The command-line autocomplete
+// then iterates the resulting collection at runtime — no central list to
+// keep in sync.
+//
+// Non-interactive one-shot dispatch arms (NEW, OPEN, SAVE, …) live in
+// `app/commands.rs` and don't have a `CadCommand` impl; they're absent
+// from autocomplete by design. Add an explicit `inventory::submit!` next
+// to their dispatch arm if you want them surfaced.
+
+pub struct CommandRegistration {
+    pub names: &'static [&'static str],
+}
+
+inventory::collect!(CommandRegistration);
+
+/// All registered command names, including aliases.
+pub fn all_registered_command_names() -> Vec<&'static str> {
+    inventory::iter::<CommandRegistration>
+        .into_iter()
+        .flat_map(|r| r.names.iter().copied())
+        .collect()
+}