docs(reference): restructure

[kfox]

PR 3 of 8 in the reference-guide review follow-through. Prose and structure only — no code, no hardware.

What changes

Two chapter titles that did not say what was in them.

• Chapter 5 Inside the Machine → The Link and the Memory Map
• Chapter 6 Everything Outside → Inputs and Outputs — which is what its filename has said all along, and what a reader scanning the contents for MIDI, WLED or recording can actually find.

Neither number moves, so every Chapter N cross-reference in either direction still lands where it did.

Chapter 5 was two books. A memory map and a write budget, then a Python extension API — contributor material sitting inside a user-facing volume, after a frame budget. "Writing Your Own Scene" and "Writing Your Own Overlay or Generator" move to a new Chapter 7, Extending c64cast (08-extending.md), with a short opener saying who it is for and pointing at docs/architecture.md for why each module is built the way it is. Appending rather than inserting keeps chapters 1–6 numbered as they are, and satisfies test_book_build's "digit chapter numbers are exactly 1..N" rule with no renumbering.

The book kept a promise it was breaking. The introduction says a subject belonging to two chapters is written once and referred to from the other; ASID and the MIDI scene were written twice, near-verbatim. Chapter 2's catalogue entries now carry what a configuration needs — the keys, the extra, the ports — and defer the mechanism to Chapter 4. Two facts that only the shrunk copies carried (multitimbral's last-note priority; where to find the ASID hosts) were moved rather than dropped.

Counts updated: 01-introduction.md, docs/reference/README.md, CLAUDE.md, and the changelog's own description of the book (which has not shipped yet, so its "Six chapters" entry is corrected in place rather than amended).

Verification

make lint                                                  # clean
uv run python -m unittest discover -s tests -q             # 3297 tests, OK
make reference-appendices && git diff --exit-code docs/    # no generated drift
make books                                                 # all three PDFs render

Read the artefact as well: the contents page lists both new titles and the new chapter, and the Chapter 7 opener page renders with its three section bullets in white on the accent page.