CI's `cargo doc -p mfsk-core --features full --no-deps` runs with
`RUSTDOCFLAGS=-D warnings`; ten pre-existing intra-doc links to
private items + a few unescaped `[N]` bracket pairs were silently
accumulating but tripped the v0.5.9 release gate. Convert each to
plain backticks (no link). No code change.

- core/dsp/fft_15.rs: escape `X[1] ↔ X[4]` brackets
- core/dsp/subtract.rs: drop link to private `generate_iq`
- core/llr.rs: drop link to non-existent `Protocol::INFO_SCRAMBLE_RVEC`
- core/sync.rs: drop link to non-existent `Protocol::SPECTRUM_WINDOW`
- ft8/baseline.rs: drop link to non-imported `Spectrogram`
- ft8/params.rs: drop link to private `decode_block::DecodeTuning`
- ft8/refine_fine.rs: drop links to private `fine_sync_power*`
- jt9/mod.rs: replace `[decode]` link with prose describing softsym path
- wspr/demod.rs: wrap inline pseudo-code block to escape `[i]` brackets

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>