fix(hint): labelled tier-3 block format + snapshot (ADR-0053 /runda)

Final /runda found the rendered block was three bare unlabelled lines,
deviating from the approved exemplar format. Fix:
- emit_tier3_block now renders a `Hint` heading + aligned `What:` /
  `Example:` / `Concept:` lines (hint.block.* labels); concept stays muted
- lock the format with an insta snapshot (hint_block_insert)
- amend ADR-0053 D2/D4 + exemplars: drop the `Next:` line (tier-2 ambient
  already owns live position-awareness — user-confirmed), align exemplars
  to the shipped format

2499 pass / 1 ignored, clippy clean.

src/friendly/strings/en-US.yaml

@@ -391,6 +391,13 @@ hint:
   # H2 / ADR-0053: shown by `hint` / F1 when there is nothing specific
   # to expand on (no recent error, empty input).
   getting_started: "Start typing a command and press F1 for a hint, or type `help` for the full command list."
+  # Tier-3 block scaffolding (ADR-0053 D4): the heading + the labels the
+  # `what` / `example` / `concept` parts render under.
+  block:
+    heading: "Hint"
+    what: "What"
+    example: "Example"
+    concept: "Concept"
   # ── Tier-3 teaching blocks (ADR-0053 D3) ──────────────────────────
   # Per-form command hints (`hint.cmd.<form>`) and per-class error
   # hints (`hint.err.<class>`), each a `what` (1–2 sentences) / `example`