docs(reference): appendix I, the optional extras

[kfox]

PR 4 of 8 in the reference-guide review follow-through. Stacked on #202 (merged), so this branches off main. Docs plus one real --doctor fix; no hardware.

What changes

The source of truth was wrong first. doctor._EXTRAS is already the name → module → what-it-unlocks table --doctor probes with, and it was missing wled — eleven entries against twelve declared extras, with nothing holding the two lists together. So a missing zeroconf, the one thing standing between [wled].listen and a WLED app that can discover the virtual device, showed up as silence in the command whose whole job is to say what is missing. The entry is added, and tests/test_packaging_metadata.py now pins _EXTRAS's names to the key set of [project.optional-dependencies] minus all, reporting both directions of drift.

Appendix I — Optional Extras, generated, joining doctor._EXTRAS with tomllib-parsed pyproject.toml: the extra, the module that has to import, what it unlocks, and its requirement strings with their bounds. Read from the file rather than from installed metadata, so a checkout that was never installed still builds the book.

The one thing a table cannot carry is in prose above it: extras do not accumulate — c64cast[midi] installed over c64cast[video] leaves you with MIDI and no video — so the install worth asking for is c64cast[all], or uv sync --all-extras from a checkout. That is the same reasoning doctor._probe_extras already prints as its hint.

The glossary becomes Appendix J (29-appendix-j-glossary.md), and the introduction gains a sentence pointing at I: the chapters have always named an extra where a feature needs one — video for a video scene, midi for a MIDI one, wled for the virtual device — and nothing collected them.

Bookkeeping: the new path in APPENDICES, the generated-file count 8 → 9 in tests/test_reference_appendices.py, and every "A to H" sentence — docs/reference/README.md and its source table, 01-introduction.md, Makefile, CLAUDE.md, README.md and the generator's own docstring. The "run this after changing…" lists gain or an install extra.

Verification

make lint                                                  # clean
make typecheck                                             # mypy --strict + pyright clean
make test                                                  # 3298 tests, OK
make reference-appendices && git diff --exit-code docs/    # no generated drift
make reference                                             # PDF renders
c64cast --doctor --skip-probe                              # EXTRAS now lists wled

Read the artefact too: the Appendix I opener page and its table render, and the contents page lists I Optional Extras and J Glossary at the right pages.