[feat] True-color internal rendering (CRY/RGB16 -> RGB888)

[JoeMatt]

Split out of #338 (candidate feature 3) so closing the epic doesn't drop the idea from the board.

Internal CRY/RGB16 -> RGB888 conversion to kill banding in gouraud shades and CRY blends (PSX "true color" analog). Output-stage only, cheap-ish, biggest visual win on shaded 3D titles (Battle Morph, IS2, Checkered Flag).

Guardrails per #338: default off, stock path bit-identical, prove pace-neutrality with the existing corpus tooling.

Not scheduled to a release milestone yet — promotion out of Icebox is a roadmap decision (see #371 working rules).

[JoeMatt]

Flagging a tracker inconsistency rather than acting on it: this issue's own body says "Not scheduled to a release milestone yet — promotion out of Icebox is a roadmap decision," but the issue is currently milestoned to v3.5.0. Per #371's working rules ("promotion out of the icebox is a roadmap decision, not a drive-by"), I'm not implementing this true-color-rendering feature or changing the milestone myself — leaving it for a maintainer to confirm which is correct (promote deliberately, or reset to Icebox to match the body).

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