feat: IonModeBasis GT3b producer payload

[uwarring82]

Summary

Producer half of the ratified cross-repo GT3b handshake with iontrap-dynamics (taskcards/TC-ion-mode-basis-payload.md). Emits a versioned, serialization-neutral IonModeBasis payload that the sibling's materialize_ion_mode_basis(...) validates and turns into its normal→local symplectic map S. This repo stays classical structural dynamics — no phase-space / S-builder math here.

What's included

• IonModeBasis record + ion_mode_basis(...) emitter (mode_basis.py), pure-functions-over-frozen-records style.
• Wire invariants fixed explicitly: ion-major/axis-minor B (3N,3N) with B[3i+c,m]=eigenvectors[m,i,c], coordinate_frame emitted as the consumer's canonical identifier (exact-match on the consumer side), frequency↔column alignment, and sign-canonicalised eigenvectors (largest-|component| positive) for reproducibility.
• D3 local-reference gauges: trap_curvature (default) and diagonal_hessian, recorded in tags.
• Serialization-neutral asdict()/as_arrays() returning detached array copies + primitive tags (no shared runtime class).
• Producer positivity/finiteness guard so ω>0 holds even for a hand-built/critical ModeResult.
• Contract test + a guarded real-consumer round-trip test (pytest.importorskip) that drives materialize_ion_mode_basis(**payload.asdict()).
• Docs (api.md), CHANGELOG.md, docs/LOG.md updated.

Verification

• ruff check ., mypy, mkdocs build --strict all clean; contract suite green.
• End-to-end against the real iontrap-dynamics consumer (equal-mass, mixed-species, both gauges, N=2/3/5): materialises, retains tags, built S symplectic to |SΩSᵀ−Ω| ≤ 1.6e-15.

Notes

• schema_version = 1 is provisional pending consumer ratification (tracked in docs/LOG.md).
• The legacy to_mode_configs() per-mode seam is unchanged and is not the GT3b handshake.

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