docs: Material 3 Expressive + advanced Nav 3 patterns

[aldefy]

Why

Two coverage gaps, confirmed by surveying the reference files (and echoed by
r/androiddev demand for M3 Expressive examples):

1. M3 Expressive was nearly absent — theming-material3.md had zero
   Expressive content; material3-motion.md mentioned MotionScheme.expressive()
   only in passing.
2. Advanced Nav 3 was missing — migration basics were solid, but the hard
   problems (results, auth-gating, transitions, adaptive list-detail, multiple
   back stacks) weren't covered.

Changes (3 existing files expanded — no new files)

• theming-material3.md → ## Material 3 Expressive:
  MaterialExpressiveTheme, the @ExperimentalMaterial3ExpressiveApi opt-in,
  and the expressive component family (ButtonGroup, SplitButton,
  FloatingActionButtonMenu/ToggleFloatingActionButton, LoadingIndicator,
  HorizontalFloatingToolbar), expressive shape/type, plus an anti-pattern.
• material3-motion.md → ## Expressive motion: spatial-vs-effects under
  the expressive scheme, why the components need MaterialExpressiveTheme, a
  review flag for wobbling fades.
• navigation-migration.md → ## Advanced Nav 3 patterns: returning
  results, auth-gated navigation, animated/shared-element transitions in
  NavDisplay, adaptive list-detail (NavigableListDetailPaneScaffold), and
  per-tab back stacks.
• SKILL.md → routing rows + trigger keywords so M3 Expressive / advanced
  Nav 3 queries land on the right file.

Accuracy

Every new API verified against current androidx docs via context7. Because the
expressive component APIs are @ExperimentalMaterial3ExpressiveApi and their
exact parameter lists shift across compose-bom alphas, each signature is shown
as shape with an explicit "verify against your BOM" note — the verified opt-in
annotation and concepts are stated as fact; unverifiable exact params are not
asserted. Nav 3 advanced APIs (NavDisplay shared-transition scope,
NavigableListDetailPaneScaffold, LocalNavAnimatedContentScope) are verified.

Verification

• check-description-length.sh → 843/1024 (frontmatter untouched).
• validate-plugin-manifest.sh → valid.

Content-only; independent of the verify-claims CI work (PR #16). Minor version
bump at release time.