Report progress while the resident video VAE decodes

[AlexanderIstomin]

The resident decoder reports progress while loading its 36 blocks, then
decodes in silence. A caller's progress display therefore holds its last
value for the entire decode.

That is not a small tail. On a still — where one frame is kept from a
decoded chunk — the decode is about a quarter of the whole generation.
In our app it presented as the progress bar sitting at 100% for two to
three minutes, and users reasonably read it as a hang.

The change

run_resident_tile reports each block it runs, and
h3_video_vae_decoder gained an optional progress sink so callers can
label the decode separately from the load. A decoder without a sink
decodes exactly as before, so this is additive: h3_video_vae_decode
and every existing caller are unaffected.

Spatial tiles each run the full block stack, so a raw per-tile count
would restart the reported figure once per tile. The decoder tracks its
place in the tiling and forwards position in the whole decode instead —
a four-tile canvas advances 1/144 to 144/144 rather than counting to 36
four times, so the figure only ever moves forward.

Measured on an M1 Pro: a 352-square still decodes in four tiles, and the
reported sequence is monotonic across all of them.