feat(ota): on-device ota branch [stable|dev|default] channel switch

[Elektr0Vodka]

Summary

Adds an on-device command to switch which OTA release channel an observer node pulls from at runtime, instead of the channel being fixed at build time.

• ota branch — report current channel + resolved manifest base
• ota branch stable|dev|default — set the channel (persisted); default = the build's native channel
• Scope: same as ota update (any admin path)

How it works

• New NodePrefs::ota_channel (uint8, def("ota_ch"), 0=native/1=stable/2=dev) — named JSON key, back-compatible with existing /prefs.json.
• src/helpers/OtaChannel.h resolves the selector to a compile-time base macro.
• build.sh bakes three bases into every observer build: OTA_MANIFEST_BASE (native), OTA_MANIFEST_BASE_STABLE (/mqtt/v), OTA_MANIFEST_BASE_DEV (/mqtt/dev/v).
• otaFromManifest() takes the base as a parameter; the ota check path and the repeater/room-server deferred-flash path resolve it from prefs.

Tests

• test/test_ota_channel (env native_ota_channel): resolver + arg parser — 6/6.
• test_config_serializer: ota_channel default / round-trip / missing-key — +3.
• Full native suite: 533/533. Repeater + room-server observer firmwares build & link; both channel URLs verified baked into the binary.

Dependency (out of scope here)

Dev-channel OTA fetch needs DutchMeshCore-OTA feat/dev-stable-ota-channels deployed (serves /mqtt/dev/v) + Cloudflare "Always Use HTTPS: Off" + bot-challenge Skip rules on /mqtt/dev/*. Stable channel unaffected.

Design & plan: docs/superpowers/specs/2026-09-14-ota-device-channel-switch-design.md, docs/superpowers/plans/2026-09-14-ota-device-channel-switch.md.

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