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/keys.rs

@@ -224,6 +224,10 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     ),
     ("hint.ambient_expected", &["expected"]),
     ("hint.getting_started", &[]),
+    ("hint.block.heading", &[]),
+    ("hint.block.what", &[]),
+    ("hint.block.example", &[]),
+    ("hint.block.concept", &[]),
     // Tier-3 teaching blocks (ADR-0053 D3) — Phase-B exemplars.
     ("hint.cmd.insert.what", &[]),
     ("hint.cmd.insert.example", &[]),