docs: v0.20.x patch — close audit-flagged doc-only gaps

[subinium]

Type: docs / patch-safe (no behavior change)
Effort: m (~10 sub-tasks, all rustdoc-only)
Discovered by: v0.20.0 design-discipline audit (CHANGELOG `Known v0.21 migration notes` last paragraph)

Background

The v0.20.0 design-discipline audit identified ~10 patch-safe doc-only gaps that did not block the v0.20.0 release but should land as a 0.20.x patch.

Sub-tasks (all additive, no behavior change)

1. Add `# Example` blocks to 16 `status.rs` family methods — `alert`, `definition_list`, `divider_text`, `badge`, `badge_colored`, `key_hint`, `stat`, `stat_colored`, `stat_trend`, `empty_state`, `empty_state_action`, `code_block`, `code_block_lang`, `code_block_numbered`, `code_block_numbered_lang`, `accordion`. (`docs/RUSTDOC_GUIDE.md` mandates Purpose + Family + Example.)
2. Add `# Example` block to `vsplit_pane` at `src/context/widgets_display/split.rs:66`.
3. Add `/// Deprecated alias for [`Self::selected_file`].` at `src/widgets/collections.rs` (clears the V4 violation).
4. Add `# Panics` sections to 8 hook methods: `use_state`, `use_state_named`, `use_state_named_with`, `use_state_keyed`, `use_state_keyed_default`, `use_memo`, `use_effect`, `animate_value`. Each cites the actual panic message text per RUSTDOC_GUIDE.md.
5. Document the `Gauge::show` / `LineGauge::show` / `Breadcrumb::show` `expect` branch as unreachable — replace the misleading panic message or use `unwrap_or_else(|| Default::default())`.
6. Add `# Family` paragraph to `Gauge` and `LineGauge` types describing how they relate to `progress` / `progress_bar` (linked patterns from RUSTDOC_GUIDE.md).
7. Cross-link `animate_bool` / `animate_value` to `Tween` and `Spring` in their `# See also` sections.
8. (Optional) Audit script V8–V12 rules — `scripts/api_audit.sh` extension proposed by the audit:
   • V8: cross-family naming asymmetry
   • V9: display-family return-shape uniformity
   • V10: f32/f64 ratio drift
   • V11: missing `# Example` on public items
   • V12: missing `# Panics` when body contains panic/expect/unwrap/unreachable

These would have caught every Critical drift surfaced in the v0.20.0 audit. Land them as a separate PR if scope grows.

Notes

All 10 sub-tasks are additive-only. The audit report is saved in the v0.20.0 design audit log (agent run `a34899af4e6a71de9`). Cap the patch at a single PR.