Implement movie mask transition opcode

docs/opcode-reference.md

@@ -1138,12 +1138,12 @@ Port status (2026-07-29): implemented as retained signed-dword graphics configur
 - **evidence:** Ghidra /v2: op_0x249_load_raw_texture_surface@0x424b20 is instruction-length 7 and is contract-identical to gfx_op_0x1f9_load_surface through release, unchanged packed operand, asset_open_indexed_entry, RGB colorkey conversion, load failure, and cleanup. Its only relevant distinction is mode-1 gfx_surface_mode1_ctor, a tiled large-image wrapper: gfx_tiled_surface_create@0x432ff0 splits logical dimensions into ordinary mode-0 child textures; gfx_tiled_surface_upload_agf@0x431a10 decodes/uploads regions; gfx_tiled_surface_blit@0x4316b0 subdivides logical source rectangles. Corpus literals include FIELD 0x32da..0x32dd -> SO005/SO007/SO008A/SO007A. The former claim that only 0x249 bypasses scene normalization was wrong because native never performs scene normalization for 0x1f9 either.
 
 ### 0x24d `play-movie-mask-transition` (play-movie-mask-transition, argc 12)
-- **summary:** Open a movie into a scratch surface, retime it to the requested duration, copy each decoded frame's green channel into a byte-per-pixel mask, and register a blocking retained-surface transition from an old object range to a newly drawn range within the supplied rectangle.
+- **summary:** Capture an old retained-object range into the newly drawn scratch surface, retime a movie to the requested duration, and use each decoded frame's green channel as a byte-per-pixel alpha mask within the supplied rectangle.
 - **grounding:** source=investigation, confidence=high
 - **depends on:** 0x236, 0x223
 - **evidence:** Ghidra /v2: op_0x24d_play_movie_mask_transition@0x424db0 opens operand 10 through movie_to_texture_open_asset_graph, configures delay operand 11, sets IMediaPosition::put_Rate to native_stop_time_ms / operand12_ms, allocates a width*height byte mask through movie_texture_allocate_transition_mask@0x415d90, and calls gfx_movie_mask_transition_register@0x47f560 with operands 1-9/11. movie_texture_renderer_receive_sample@0x4628d0's mask-active branch copies byte +1 of every bottom-up RGB24 sample pixel, i.e. its green channel, directly into that mask. The retained compositor's type-1 branch applies the movie-updated mask between the old and new object ranges and holds its blocking dirty state until playback completes. Both DEBUG.BIN sites use TEST.AGF (an MPEG program stream), delay 0, and duration 1000 ms.
 
-The RGB24 sample callback copies the bottom-up green byte verbatim. In the native 32-bit mask compositor, captured RGB remains unchanged and the output alpha byte is the high byte of `((source_argb >> 8) * mask_byte)`; this records the exact packed-integer operation, including its low-color carry, rather than approximating it as a scalar luminance crossfade. Mode 1 initializes the mask to 0 and completes at 255; all other modes initialize at 255 and complete at 0.
+The RGB24 sample callback copies the bottom-up green byte verbatim. In the native 32-bit mask compositor, captured RGB remains unchanged and the output alpha byte is the high byte of `((source_argb >> 8) * mask_byte)`; this records the exact packed-integer operation, including its low-color carry, rather than approximating it as a scalar luminance crossfade. Mode 1 initializes the mask to 0 and completes at 255; all other modes initialize at 255 and complete at 0. Port status (2026-07-29): implemented through the existing retained-range software rasterizer and dynamic scratch-surface path. The decoder honors start delay and native stop/duration retiming, publishes logical top-down green masks, and holds opcode 0x21c until movie completion; click completion remains limited to type-0 transitions.
 
 ### 0x24e `set-gfx-animation-service-flags` (set-gfx-animation-service-flags, argc 1)
 - **summary:** Replace the retained graphics animation-service flags with operand 1. BTL brackets combat presentation with values 1 and 0; GAMECLEAR uses 3 and 0.