Add grounded Python typing docs for serialization and async iteration

[citadelgrad]

Summary

Adds grounded Python typing coverage for two missing areas:

• T64-async-iteration: async iteration/generator technique doc with pyright-checked snippets and source anchors.
• UC19-serialization: serialization safety use case for parsing JSON/wire data into typed payloads, with TypedDict/dataclass/Literal patterns.
• Updates Python navigation, cross-language taxonomy counts, and Python source references.
• Fixes an existing strict-Pyright docs-snippet failure in UC11-effect-tracking.md by annotating @contextmanager examples with Generator instead of Iterator.

Grounding / sources checked

Fetched and used official/source docs for Python typing, collections.abc, json, PEP 492, PEP 525, PEP 589, PEP 655, PEP 705, PEP 681, Pydantic serialization, and mypy TypedDict behavior. These are listed in taxonomy/sources.md and in the new docs' Source anchors.

Review passes

• Research grounding review: source-backed claims only; Python UC19 is marked ~ in the taxonomy because full serialization safety is achievable via patterns/libraries, not native compiler derivation.
• Python typing/code review: snippets are pyright-verified; async generator annotations and contextmanager annotations match current checker behavior.
• Documentation integration review: navigation, SKILL.md, README counts, and taxonomy coverage were updated together.

Annotation reasoning: @contextmanager uses Generator, not Iterator

The UC11-effect-tracking.md change from Iterator[T] to Generator[T] is intentional and checker-driven:

• A function decorated with @contextmanager is implemented as a generator function: it yields the value exposed by with ... as value.
• Iterator[T] is broad enough for some checkers, but current Pyright strict mode reports that annotation as deprecated for @contextmanager and asks for Generator[T]. This is not limited to Python 3.14; it appears under the repo's strict Pyright config across Python targets 3.10-3.14.
• ty 0.0.56 accepts both Iterator[int] and Generator[int] and reveals the with binding as int in both cases.
• Therefore Generator[T] is the more portable documentation choice for this repo: it passes strict Pyright while remaining accepted by ty, and it more precisely describes the generator function before contextmanager wraps it.

Review follow-up

Follow-up commit 5bf502f addresses reviewer findings: Python navigation now exposes all 33 catalog entries and UC21, UC19/T64 snippets in the touched docs have zero skipped snippets, the shared T64 taxonomy wording is language-neutral, and the TypeScript technique count is corrected to the filesystem-backed 35.

Verification

• make check
  • tenets sync: all skills ok
  • snippet extractor tests: 39 passed
• make verify FILE=plugin/skills/python/catalog/T64-async-iteration.md
  • 2 ok, 0 fail, 3 expected-fail, 0 missing-expected-error, 0 skipped, 0 tool errors
• make verify FILE=plugin/skills/python/usecases/UC19-serialization.md
  • 1 ok, 0 fail, 4 expected-fail, 0 missing-expected-error, 0 skipped, 0 tool errors
• make verify-python
  • checked 54 files: 0 with failures, 0 tool-errors