Transmission AOD source modes: manual versus OpenAQ-derived PM2.5 estimate

[Jesssullivan]

Linear: https://linear.app/tinyland/issue/TIN-1728/transmission-aod-source-modes-manual-versus-openaq-derived-pm25

Problem

Once OpenAQ PM2.5 estimates can drive AOD550, the transmission sheet needs an explicit source model. Users should be able to tell when AOD is manual versus derived from a clicked-point PM2.5 kernel estimate, and manual edits should not be silently overwritten by subsequent map events.

Scope

• Add explicit source modes for AOD: manual, clicked-point PM2.5 estimate, and future viewport/center estimate.
• Show provenance for OpenAQ-derived AOD, including PM2.5 value, confidence, station support, and conversion note.
• Provide an explicit detach/manual action when a modeled source is active.
• Avoid overwriting manual AOD after user edits unless the modeled source mode is explicitly enabled.

Acceptance

• Manual AOD survives pan/tap/layer refresh unless the user opts into modeled source mode.
• OpenAQ-derived AOD shows source provenance and sparse-confidence warnings.
• Tests cover manual detach, modeled attach, and no-reading fallback.

Related: #253, #275, #271, #250.