feat(hint): H2 Phase D — coverage gate, F1 strip, status flips (ADR-0053)

Completes H2:
- comprehensiveness coverage tests: every REGISTRY command form has a
  hint_id resolving to a hint.cmd.* block, and every runtime error class
  resolves to a hint.err.* block (enforces ADR-0053 D6)
- ADR-0051 keybinding strip advertises F1 in the editing (leads) and
  default states; +shortcut.hint label; 12 full-panel snapshots
  re-accepted (status-bar line only)
- flip ADR-0053 -> implemented, requirements H2 + A1 -> [x], README

2498 pass / 1 ignored, clippy clean.

docs/requirements.md

@@ -250,16 +250,13 @@ since ADR-0027.)
 
 ## App-level commands (per ADR-0003)
 
-- [/] **A1** All canonical app-level commands implemented and
+- [x] **A1** All canonical app-level commands implemented and
   available in both modes: `save`, `save as`, `load`, `new`,
   `rebuild`, `export`, `import`, `seed`, `replay`, `undo`,
   `redo`, `mode`, `help`, `hint`, `quit`.
-  *(Partial: **14 of 15** implemented and available in both modes —
-  `quit`/`q`, `mode simple|advanced`, `help`, `save`, `save as`,
-  `load`, `new`, `rebuild`, `export`, `import`, `replay`, `undo`,
-  `redo`, and now **`seed`** (ADR-0048 / SD1, done 2026-06-11).
-  **Only `hint`** (tracked as H2) remains unregistered. A1 closes
-  when H2 lands.)*
+  *(Done 2026-06-15: the last command, **`hint`**, landed with H2
+  (ADR-0053). All 15 canonical app commands are now registered and
+  available in both modes.)*
 
 ## DSL data commands
 
@@ -793,8 +790,16 @@ since ADR-0027.)
   `returning `) still shows the raw expression first-set —
   typing-time completion already offers the right candidates
   there, so the payoff is small.
-- [ ] **H2** `hint` provides contextual help for the current
+- [x] **H2** `hint` provides contextual help for the current
   input or the most recent error.
+  *(Done 2026-06-15, ADR-0053: an **F1** keybinding gives a tier-3
+  teaching hint for the live partial input (read-only overlay), and a
+  submitted **`hint`** command expands on the most recent runtime error.
+  A new `hint.cmd.<form>` / `hint.err.<class>` catalogue tier
+  (`what`/`example`/`concept`) covers every command form + the 9 runtime
+  error classes, enforced by a comprehensiveness coverage test. Deferred:
+  the pre-submit-diagnostic route + `diagnostic.*` blocks (#38),
+  clause-concept hints (#37).)*
 - [x] **H3** `help` provides general reference and per-command
   help.
   *(Done 2026-06-07: the **general reference** is `help` (no arg) —