Yaesu FTM-300DR support via microSD (#48)

[ww8l]

Closes #48.

Programs the Yaesu FTM-300DR from its own microSD backup (FTM300D/BACKUP/CLNFTM300D.dat): the app patches the file in place (keeping CLNFTM300D.dat.orig), and the radio loads it with SD CARD → BACKUP → READ FROM SD → ALL.

What's in it

• Memories: radios/yaesu_ftm300/. 999 records, names, and the name-length code. The layout started from CHIRP's ftm300d.py and was corrected against the radio: the offset is a u16, byte 11 carries the user CTCSS, the name-length table is a length code, and DCS is 0-based.
  • Re-encoding Tim's own 119 channels: 88 come out byte-identical, and the other 31 differ only in skip, M-GRP, clock shift and VW, which have no database column.
  • Through the dev export path: 61 of 62 channels written (the D-STAR channel excluded), and nothing outside the memory tables moved.
• Settings: 147 fields. One spec generates both the Rust table and the form schema. They were measured with RT Systems writing the SD file: a 16-pass bit signature, an enum sweep and a text pass.
  • Decoding Tim's file reproduces what RT Systems displays for it.
  • Five fields where RT Systems and CHIRP disagreed were settled on the radio's own screen: Band Scope, Sub Band Mute, Date Format, Digital Popup "Continue", and Com Port speed/output.
• M-GRP: a per-codeplug channel checklist on the codeplug page.
  • Stored in codeplug_memory_group (migration 0020).
  • Reaches file exporters as ExportRequest::memory_group.
  • Written as the record's M-GRP bit.
• Wiring: card discovery, Program dialog steps, the profile-editor settings reader, the seed row and the README.

Hardware (Tim's FTM-300DR)

• ✅ Identity read
• ✅ One-byte name write: the radio accepted the edited file, so there is no checksum
• ✅ Full 61-memory codeplug written through the app's Write to SD card
• ✅ Band probe
• ✅ Settings spot checks
• ⬜ M-GRP after an app write: not yet checked on the radio. Tim will file an issue if it doesn't work.

Known gaps

• Airband 1.67 / 6.67 kHz offsets display 417 Hz off; the radio's fine-step code for these isn't measured yet.
• Com Port 57600 is withheld.
• Settings not exposed: FM bandwidth / RX mode, Target Location, Clock Type and GPS Device. They aren't in RT Systems' dialog.

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