Adversarial review found a documentation defect, not a code one: both
docs/decisions.md and the WatermarkSelector comment asserted the deco path
was unaffected by this change. That is true of the *resolution* half and
false of the *routing* half. SelectWatermarks puts deco-derived options
into the same list the routing guard filters, so a deco watermark whose
resolved path is a URL is rerouted to the graphics engine too — including
the generated-initials localhost URL, which only the deco path still emits
and which plausibly rendered through ffmpeg before.
That reroute is intended (routing by what the path is beats routing by
provenance, which would drift), so the fix is to say so accurately rather
than to narrow the guard. Also records the accepted per-frame cost
asymmetry the entry previously argued on correctness grounds alone.
The guard is extracted as CanUseFFmpegNativeWatermark so it can be tested
directly — review's highest-value gap was that the half of the fix which
decides whether pixels appear had no automated coverage, only the one-off
live E2E. Nine cases pin it, including the localhost-fallback reroute.
Both deferrals now point at real issues instead of an unverifiable
"tracked separately": #510 (deco vs precedence-level missing-logo policy)
and #511 (remote-fetch hardening — timeout, size cap, redirects, pooling,
caching, SSRF).
Also pins scheme-case insensitivity in the selector.
