docs(readme): comprehensive rewrite — typed scalars, spec quirks, full TOML reference

- Add a "What the generated types look like" tour with real outputs:
  typed scalars (chrono / url / bytes / uuid), typed BTreeMap for
  additionalProperties, constraint doc-comments (and the no-runtime-
  validation invariant), tagged unions, the new untagged fallback for
  hybrid string-or-object unions, extensible enums with Custom(String)
  fallback, and per-operation typed errors.
- New "Spec-quirk overrides" section documenting nullable_overrides,
  the new [extensible_enums] knob, type_mappings, and the
  [generator.types.strategies] per-format opt-out.
- Expand TOML reference to cover all sections that ship today
  (streaming endpoints, retry, tracing, headers, registry, types).
- Note coverage: 54 real-world specs compile cleanly under
  scripts/spec-compile.sh.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lightsofapollo