Reference RatioTap from the README, book, and comparison doc

[tap]

What this changes

Adds the missing back-references to RatioTap: a "Position in the Tap family" section in the README (family diagram, clock-topology decision rule, composition recipe), a redirect in the Limitations bullet, a RatioTap mention where the book's survey chapter recommends synchronous resamplers, a new Part V subsection on the 44.1 ↔ 48 composition, and one line in docs/COMPARISON.md so the book's paraphrase of it stays accurate.

Why

The relationship between the two rate converters was documented entirely one-directionally. RatioTap's README carries the family diagram, the "boundaries are identity" delegation to this library, the bluetooth_bridge composition example, and a test-only submodule of this repo for golden cross-validation — while this repo never mentioned RatioTap (or DspTap) outside header comments. Two places actively sent readers elsewhere: the README's Limitations ended at "No 44.1 ↔ 48 kHz conversion" with no pointer to the sibling built for exactly that, and book/src/part0/two-crystals.md recommended soxr/libsamplerate for genuine conversion — text that predates RatioTap shipping.

The Part V addition is deliberately prose rather than a code walk, and says so in the text: the book's include-anchor contract quotes only from this tree, and including RatioTap source would require submoduling it back — a cycle against its test-only submodule of this repo. The working recipe stays RatioTap's; this book contributes the decision rule (which engine applies is a property of the clock topology, never inferred from a float ratio).

Verification

• Pinned mdBook v0.4.40 (same digest CI verifies) builds the book with zero warnings/errors — the exact book job gate.
• The image-reference check from ci.yml passes.
• Every quoted figure was checked against RatioTap's current README (v0.2): 160/147 and 147/160 ratio pair, −109 dB / −99 dB cross-validation over every phase, bluetooth_bridge's +200 ppm / 997 Hz / 2.0 ms; the ±1.5 × max_deviation_ppm servo clamp was verified against include/srt/pi_servo.h.
• Not run locally: the full CI matrix — this is a docs-only change (README, two book chapters, one comparison-doc bullet); no code, tables, or figures touched, so the icount ratchet and docs-freshness gates are unaffected by construction. The appended "Verify it yourself" recipe (clone RatioTap, build bluetooth_bridge) was checked against RatioTap's examples/CMakeLists.txt target name but not executed here.

Notes for the reviewer

• The docs/COMPARISON.md edit is the one file beyond the README/book scope, touched only because two-crystals.md characterizes what that document says; updating one without the other would make the book's paraphrase false.

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