The vendored rate controller packs its wavelet block index into 16 bits
(RDOperation.block_offset_saving), so a mode whose 32x32-block count exceeds
u16::MAX wraps inside the controller and corrupts the bitstream — ~8K-class
4:4:4 territory. Compute the exact count (`block_count_32x32`, the counting walk
of upstream init_block_meta, pinned against the validated Apple WaveletLayout)
and expose `pyrowave_mode_fits_rdo`; the negotiator downgrades such a session to
4:2:0 before the Welcome (the honest-downgrade channel), and both encoders
refuse outright if one slips through rather than emit a wrapped stream.
Vendor patches: 0002-rdo-saving-clamp (analyze_rate_control.comp clamps the
saving accumulation to the target, same overrun class as 0001; slangmosh.hpp
regenerated), 0003-devel-encode-16bit-read (devel tool y4m 16-bit plane reads;
tool-only, kept so the vendored source stays honest).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
enricobuehler
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