feat(chirp): DCS / Dtcs cross-mode import and export mapping

[pskillen]

Problem

CHIRP import/export shipped in #103 and model-only round-trip landed in #110. CTCSS tones round-trip via Tone / TSQL + rToneFreq / cToneFreq → internal Channel.rxTone / Channel.txTone.

DCS is not wired through the boundary. Today:

CHIRP column	Import	Export
DtcsCode	ignored	constant 023
DtcsPolarity	ignored	constant NN
RxDtcsCode	ignored	constant 023
CrossMode	ignored	constant Tone->Tone

parseChirpTones() only handles Tone and TSQL CTCSS modes. formatChirpToneFreq() maps DCS-shaped internal values (D023N, …) back to the CTCSS default 88.5 on export.

Committed test-data/chirp/ fixtures use CTCSS only — no DCS regression coverage.

The internal model already supports DCS (ChannelTone = CTCSS Hz or D023N-style codes; see src/lib/channelFields/tones.ts, OpenGD77 RX Tone / TX Tone mapping). The gap is CHIRP wire encoding, not a new model field.

Intended outcome

Map CHIRP DCS / cross-mode columns ↔ rxTone / txTone at the import/export boundary (no wire stash — same rules as #110).

Import

• Extend parseChirpTones() (or sibling helpers) to read Tone, DtcsCode, DtcsPolarity, RxDtcsCode, and CrossMode per CHIRP semantics.
• Map to internal ChannelTone values (e.g. D023N).
• Document supported CHIRP tone modes and lossy edges in docs/reference/chirp/channels.md.

Export

• Derive Tone, rToneFreq, cToneFreq, DtcsCode, DtcsPolarity, RxDtcsCode, CrossMode from rxTone / txTone (inverse of import).
• Replace hard-coded constants in src/lib/export/chirp/channelWire.ts.
• When tones are none, continue emitting the same constant defaults as today’s CTCSS-only fixtures (023 / NN / Tone->Tone).

Tests

• Add at least one test-data/chirp/ fixture row (or minimal bundle) exercising DCS and/or cross-mode.
• Unit tests for wire ↔ model mappers; extend round-trip compare if fixture is committed.

References

• Current mapping table: docs/reference/chirp/channels.md
• Import: src/lib/import/chirp/channelWire.ts
• Export: src/lib/export/chirp/channelWire.ts
• Outstanding log: docs/features/import-export/chirp/chirp-outstanding.md
• Parent work: #103, #109, PR #110
• CHIRP column semantics: https://chirp.danplanet.com/projects/chirp/wiki/Home

Out of scope

• New internal model fields (use existing rxTone / txTone).
• wireColumns or other provenance stash for round-trip.
• DMR columns (URCALL, RPT1CALL, …).

Manual verify

1. Import a CHIRP CSV with DCS-encoded channel(s) → RX/TX tone fields show DCS in channel detail/list.
2. Export with same profile → DtcsCode / CrossMode / polarity columns match source.
3. Edit DCS tone in CRUD → export reflects change on wire.