feat(core): carry chord harmony fn + template voicing on the wire (§6.3.1, §6.6)

[byrongamatos]

Part of #334

PR 1 of 3 (stacked) implementing the per-chord harmony annotations from feedpak 1.7.0 (FEP #24). This PR is the core wire carry; renderer (PR2) and editor authoring (PR3) follow.

What

Two OPTIONAL per-chord harmony annotations, mirroring the teaching-marks (fg/ch/sd) wire work (#536):

• Chord.fn (chord instance, §6.3.1): {rn: str, q: str, deg: int 0..11} harmonic-function object. Key-dependent, so it rides the instance. Validated by _validate_fn on both decode and emit — a partial or out-of-range fn (which would fail the schema's required-keys rule) never rides the wire. Default-omitted, mirroring bend bnv.
• ChordTemplate.voicing (chord template, §6.6): key-independent voicing-type string ("open", "triad", "shell", "drop2", "barre", …). Emitted only when non-empty; non-string wire values fall back to "".

Honesty rule

Display/teaching only. Nothing here is wired into scoring / NoteVerifier.

Deferral: fn auto-derivation = carry-only

The spec requires all three keys when fn is present. deg could be derived from the chord root, but a complete rn/q needs real chord-quality (interval) analysis — out of scope — and a deg-only fn would be schema-invalid. So server.py is unchanged: it carries author-provided fn and derives nothing. Author-provided fn always wins by construction. caged/guideTones from the FEP are also deferred per the spec.

GP import is unchanged — GP files have no reliable per-chord function/voicing.

Tested locally

• pytest tests/test_song.py → 134 passed (round-trip, omit-when-default, malformed/partial/out-of-range → None, emit-side validation of directly-constructed chords, voicing fallback).
• Independent Codex review (read-only, diff-only): caught one P3 (emit path didn't re-validate fn) — fixed by validating on emit too; re-review clean.
• The pre-existing unrelated tests/test_diagnostics_redact.py::test_song_filename_redacted_consistently failure on clean main is not touched here.

CI may be red on infra; the above was verified locally.

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