t27f: pixel sweep -- no overlay on any mode 5 crossfade frame, every lesson, both languages, three voices #10651

Boundary

A measurement on the Railway lab of specs/t27f/lesson_reel.t27 in LR_MODE=5, as on #10647. If it finds an overlay on a crossfade, the fix goes in lesson_reel.t27 (and its seal) in a follow-up PR. No gate, cap or ledger change.

User Scenarios

• A viewer watches any lesson reel. At each of the two cuts the scenes crossfade for 16 frames, and no caption, tag, card or address is drawn over the blend.
• t27f: R4e -- mode 5 cards stay inside one scene, token_floor_mux takes a caller window (Closes #10628) #10647 proved this with pixels for lesson 1, English, one voice (1300/4200/6800 ms), in the card band only. The other 26 lessons, Russian, other voices and the rest of the frame are proved only by frame arithmetic (the grid test and cmp).

Requirements

• Render every lesson (27) x both languages x three voices (1300/4200/6800, 900/2600/4100, 700/4200/8000) in the tall orientation, lossless, from solid-colour clips. Render lesson 0 in both orientations.
• On every crossfade frame (start = ceil(offset x 30), 16 frames), count the distinct colours of the whole frame. A blend of two solid clips is one colour; more than one means something is drawn over it.
• Negative control: the t27f: R4f -- mode 5 holds the address for REEL_CTA_MIN_MS, cut 1 capped at BEAT_CUT1_LAST_MS (Closes #10629) #10632 binary on lesson 1's voice must show the last card on crossfade 1 (the bug t27f: R4e -- mode 5 cards stay inside one scene, token_floor_mux takes a caller window (Closes #10628) #10647 fixes).

Success Criteria

• The sweep's table: renders, crossfade frames checked, frames with more than one colour, per language and voice.
• 0 flagged frames with the t27f: R4e -- mode 5 cards stay inside one scene, token_floor_mux takes a caller window (Closes #10628) #10647 binary, and more than 0 with the t27f: R4f -- mode 5 holds the address for REEL_CTA_MIN_MS, cut 1 capped at BEAT_CUT1_LAST_MS (Closes #10629) #10632 binary on the control.
• Any flagged frame on t27f: R4e -- mode 5 cards stay inside one scene, token_floor_mux takes a caller window (Closes #10628) #10647 gets its own fix PR before this issue closes.