docs: split architecture.md by topic + fix stray-tilde strikethrough

[kfox]

architecture.md had grown to 250 KB in a single file, with 15 paragraphs over 2,500 characters — the worst a single 14,228-character bullet cataloguing every generator. This splits it into eight topic files under docs/architecture/ and restructures the prose, preserving all content.

The routing index

docs/architecture.md becomes an index: a topic list plus an alphabetical module table.

That shape is deliberate. Roughly 30 source docstrings refer to "the <module>.py section of docs/architecture.md" as prose, so the index keeps every one of those landing correctly with no code changes.

Topic file	Modules
hardware-io.md	api.py, teensyrom_dma.py, BASIC startup
audio.md	audio.py, sampler.py, dsp.py
video-color.md	video.py, modes.py, rolling_palette.py, palette.py
scenes.md	scenes.py, composable scenes, overlays/, interstitial.py
sid.md	voice_scope.py, SID player PRG, waveform.py, midi_scene.py, asid.py
control.md	keyboard.py, vision.py, control_plane.py, midi_control.py, transport.py, midi_setup.py, camera.py
wled.md	wled_sync.py, wled_device.py, wled_sink.py
config.md	paths.py, config.py, cli.py, recording_metadata.py, ensemble.py, orchestrator.py

How content preservation was verified

Two mechanical checks rather than eyeballing:

1. The split was verified lossless before any prose was touched, by asserting each of the 35 original section bodies appears verbatim in exactly one topic file.
2. The rewrite was checked by diffing every backticked identifier per file. The only ones absent are deliberate rephrasings — status words moved to bold, formulas into code fences, values into tables.

Prose changes are structural only: giant paragraphs became subsections with lead sentences, nested parentheticals became their own statements, measurement runs became tables, and bold was cut back to genuine emphasis. Every measurement, rationale, and dead end is kept — including superseded designs and the "an earlier revision did X and it was a user-visible bug" passages — per the working rule that those are the reference's main value.

audio.md actually got slightly longer (39.6 KB → 42 KB), which is what content-preserving restructuring looks like: the added length is headings and sentence breaks, not new claims.

Bugs found along the way

Bare tildes were rendering as strikethrough. GFM treats a single ~ as a strikethrough delimiter, not just ~~. Tildes meaning "approximately" pair up across a line and strike everything between — one line carried 12 of them, producing 6 struck spans of real technical content. Converted 96 occurrences to ≈ across architecture.md, caveats.md, usage.md, troubleshooting.md, and README.md. Only genuine ~/ home paths remain.

14 dead cross-references removed. Pointers to notes that do not exist in this repo, in both [[slug]] and `slug` form; the useful content is folded into the surrounding sentence. Every [[scenes]] / [[scenes.overlays]] / [[midi_control.cc_map]] was left alone — those are TOML array-of-tables syntax, not links.

6 broken links repaired. Five were written repo-root-relative but live inside docs/, so they 404 when rendered; plus one pre-existing broken anchor in troubleshooting.md.

Verification

• make lint clean, 2,633 tests pass.
• Every link and anchor across docs/, README.md, and CLAUDE.md resolves (checked programmatically, including fragments).
• No paragraph over 2,000 characters remains in any topic file.
• CLAUDE.md's subsystem table now links directly into the topic files.

Docs only — no code changes.