ADR-0022 stage 8a: non-modal keyword completion + Esc/Backspace undo

Per the user's framing decision: there is no "completion
mode." Tab is just an action that consumes whatever is
expected at the cursor, and the existing always-on hint
panel (stage 5) tells the user what's available.

New `completion` module: `candidates_at_cursor(input,
cursor)` returns a `Completion { replaced_range,
partial_prefix, candidates }` based on the parser's
expected-token set at the cursor position. Filters to bare
keyword candidates only (no punctuation, no descriptive
labels), narrowed by the typed prefix (case-insensitive).

`LastCompletion` memo struct on `App::last_completion`
carries the cycle state: inserted_range, original_text,
candidates, selection_idx. Wrap-around forward/backward
indices.

App key handling (added before the existing matcher):
  - Tab → cycle forward if memo present; else insert first
    candidate; create / advance memo.
  - Shift-Tab → cycle backward if memo present; else
    insert last candidate (alphabetically) so the user can
    jump to the end without cycling through everything.
  - Esc / Backspace while memo alive → restore
    original_text in inserted_range, place cursor at the
    pre-Tab position, clear memo.
  - Any other key → clear memo, then process normally.

The user's symmetry preference was load-bearing here:
"insert with one keystroke, remove with one keystroke."
Both Esc and Backspace honour that — multiple Tab cycles
collapse into one undo. Documented inline.

A single-candidate completion still creates a memo so
Esc/Backspace can undo it. Multiple Tabs in a row cycle
through the candidate list with wrap-around at both ends
(per the user's #2).

Tests: 728 passing, 0 failing, 1 ignored (705 baseline →
+23: 13 completion module + 10 app integration tests
covering Tab, Shift-Tab, cycling, wrap-around, Esc-undo,
Backspace-undo, multi-Tab-then-Esc, memo invalidation by
typing or cursor movement). Clippy clean.

Stage 8b will add multi-candidate hint-panel rendering
with scroll markers (`<` `>`) per the user's #2. Stage 8c
will plumb in identifier completion + invalid-identifier
detection.

src/lib.rs

@@ -9,6 +9,7 @@ pub mod action;
 pub mod app;
 pub mod archive;
 pub mod cli;
+pub mod completion;
 pub mod db;
 pub mod dsl;
 pub mod event;