Support transparent full-canvas HD rounding and rebuild runtimes

Accept strict common-scale integer rounding for complete transparent PNG canvases while retaining source-aware alpha checks and existing opaque/exact cache identities. Add separate cache identity for newly eligible rounded transparent frames and matching Rust/Python regression coverage. Bundle rebuilt Linux and GNU Windows executables against the unchanged reviewed4978 asset pack.

Validation: 31 Rust HD tests, 7 Python audit tests, and 341 portable tests passed; zero failures. Original-data integration validated all4978 remasters at scales1–4 with bounded caches. Three fresh software SDL1920x1080 menu/mission captures are pixel-identical to published controls. Windows PE/dependency and complete shared data hashes verified; fresh Wine asset and45-mod validation passed. Artwork geometry remains independently reviewed; no held candidate is admitted by the format change.

docs/hd-streaming.md

@@ -6,7 +6,7 @@ The runtime can register the complete original picture catalog without retaining
 
 Registration does not imply a PNG has been decoded. `registered_count()` reports source-checked candidates that have not been rejected; `validated_count()` and the compatibility alias `coverage()` report frames whose artwork has passed validation. First use checks PNG format, crop bounds, matching aspect ratio, at least 2× logical resolution, visible pixels and source-aware transparency. Transparent originals require transparent negative space; opaque originals accept opaque art. A failed frame returns `Ok(None)` from `sprite`, records one diagnostic and keeps native fallback until the pack is reloaded.
 
-A fully opaque original may also use a complete, fully opaque PNG whose dimensions result from rounding one common scale by less than half a pixel on each axis. This accepts a generated 1,586×992 background for a 320×200 original without discarding its border. It does not permit partial crops, transparent artwork, square canvases with unequal axes, or arbitrary aspect changes. Every raw alpha value is checked before downsampling; rounded-frame cache entries use a separate identity and repeat the opacity check.
+A complete PNG for either an opaque or transparent original may use dimensions produced by rounding one common scale by less than half a pixel on each axis. The strict rule is `2 * abs(width * logical_height - height * logical_width) < logical_width + logical_height`; logical dimensions still equal the original, and both axes must supply at least 2× resolution. For example, 1,586×992 fits a 320×200 background, and 1,402×1,122 fits a transparent 40×32 sprite. Selection must cover the whole PNG, with no crop or an explicit full-canvas crop; inexact partial crops, conflicting half-way ties and arbitrary aspect changes remain rejected. Rounded opaque originals still require every raw PNG alpha value to be 255 before downsampling. Transparent originals retain zero-alpha negative space and at least one visible pixel with alpha ≥128, checked on both raw and resident RGBA and again on disk-cache reads. Exact-frame and rounded-opaque cache keys stay unchanged; newly eligible transparent rounded frames use a separate namespace. This dimension rule adds no per-asset policy and does not approve source contours or reconstruct missing artwork.
 
 ## Optional frame presentation policies