Fix coding rate on Short and Medium presets, add LongTurbo

[KMX415]

Summary

The preset table in src/radio/presets.py shipped with coding_rate=4/8 on ShortFast, ShortSlow, MediumFast, and MediumSlow. Per the Meshtastic Radio Settings spec (and RadioInterface.cpp in upstream firmware), only the Long Range presets and the deprecated VeryLongSlow use 4/8. The other six all use 4/5.

The wrong value cascaded through the dashboard preset dropdown into local.yaml on every Meshpoint that picked one of the affected presets, and from there into every TX (NodeInfo broadcasts, DM ACKs, relay). 4/8 is roughly 60% more airtime per byte than 4/5. RX is unaffected since the SX1302 reads CR from the LoRa header.

Changes

• src/radio/presets.py corrects CR to 4/5 on the four Short and Medium presets, and adds the missing LONG_TURBO (SF11 / BW500 / CR 4/8).
• src/transmit/tx_service.py adds (11, 500) -> "LongTurbo" to PRESET_DISPLAY_NAMES so MQTT topics and the channel-name resolver label LongTurbo consistently instead of falling through to Custom.
• src/config.py cosmetic dataclass-default fix (4/8 -> 4/5); default.yaml was already correct so this only affects code-readers and direct RadioConfig() instantiations in tests.
• tests/test_radio_presets_canonical.py (new) locks every preset's (sf, bw, cr) tuple to the upstream spec so this never regresses silently.
• tests/test_tx_service.py adds test_longturbo and expands the test_all_presets_present expected set.
• docs/COMMON-ERRORS.md documents the symptom and the dashboard re-pick / yaml edit fix path for existing installs.

No version bump

src/version.py stays at 0.7.1. The orange-triangle update indicator does not fire. Existing installs keep their cached CR until the user re-picks the preset on the dashboard or edits local.yaml. This change folds into the v0.7.2 web-auth release blurb when that ships.

Test plan

• Local: python -m pytest tests/ -q — 277 passed
• Local: ruff check src/ tests/ — clean
• CI on this PR (Ubuntu 3.12 ruff + pytest)
• Hardware on RAK V2 .141:
  • sudo git fetch origin && sudo git checkout fix/preset-coding-rate && sudo systemctl restart meshpoint
  • Open dashboard, switch preset to MediumSlow (or any of the four corrected presets), confirm CR readout shows 4/5 (was 4/8)
  • Save Radio, apply restart, confirm local.yaml now has coding_rate: "4/5"
  • Watch the next NodeInfo broadcast log line for the lower expected airtime
  • Switch to LongTurbo, confirm it appears in the dropdown with BW500 / SF11 / CR 4/8 readout
• Restore .141 to main after merge

Reported by Chiumanfu on Discord while setting up a Vancouver Meshpoint on MediumSlow.