iPhone HEIC upload: wasm-decoder go/no-go — gated on MARS-508 instrumentation (userAgent + detail.diag container) format-mix data

MARS-508 Part 3 (deliberately held). Now that 75c6ba1/v2.40.0 captures userAgent + true magic-bytes container (detail.diag) on image-upload rejects, wait for a few real HEIC decode-fails to land, then db-mars-cc queries the device/format split. Decision: add a client-side HEIC decoder (heic2any/libheif-wasm) to auto-convert instead of reject, vs keep reject+honest-message. Also finalize the remediation toast wording per the confirmed iOS-vs-Android split. Mars-local; flag pmmaster for pluto/venus inherit once sized.