fix: align EU and beacon frequency slots

[RCGV1]

Summary

• align the EU_866 Lite profile with the firmware's 865.6–867.6 MHz plan, 400 kHz spacing, and 37.5 kHz padding
• resolve beacon-offered channels through the firmware default slot on regions that define one
• add regression coverage for EU_866 centers and ham-region Add-versus-Switch behavior

Root cause

EU_866 retained obsolete frequency bounds with no Lite-profile spacing or padding. The beacon join comparison used the hash-only slot helper even after the connected-radio side became default-slot-aware, producing a needless Switch recommendation for matching ham meshes.

Validation

• xcodebuild test -project Meshtastic.xcodeproj -scheme Meshtastic -destination 'platform=iOS Simulator,id=5669CB03-8FA2-4A97-BE04-A8566204116D' -only-testing:MeshtasticTests/LoRaChannelCalculatorTests -only-testing:MeshtasticTests/BeaconAddVsSwitchTests

Fixes #2091
Fixes #2092

Summary by CodeRabbit

• Bug Fixes

  • Improved beacon channel matching by honoring firmware-defined default slots when no explicit channel is provided.
  • Updated EU866 frequency calculations to use the correct regional frequency range and spacing.
  • Improved join decisions when a device is already configured for the beacon’s default slot or a different slot.

• Tests

  • Added coverage for EU866 LiteFast frequency calculations and beacon join scenarios.

[coderabbitai]

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[garthvh]

Verified locally before merging (CI doesn't run tests):

• EU_866 now matches the firmware's Lite plan exactly — the values here are the ones worked out in #2091 from RDEF(EU_866, 865.6f, 867.6f, ..., PROFILE_LITE) (spacing 0.4, padding 0.0375; LiteFast slot 1 → 865.700 MHz).
• Beacon-side slotForChannelName now resolves through the region default slot, closing the ham-band Add-vs-Switch gap from #2092 — matching-preset beacons on default-slot regions now correctly offer Add instead of a needless reboot.
• Ran LoRaChannelCalculatorTests, BeaconAddVsSwitchTests, and ModemPresetFrequencySlotTests on the branch: 28 tests, all green — no regression to the existing slot math or beacon decisions.

Fast turnaround on both follow-ups — thanks!