fix: resolve OBJ sprite-vs-sprite priority by OAM index

[eduardovra]

Problem

In Super Bomberman 5, the menu cursor (the bomberman head, OAM index 0, priority field 2) was drawn behind the character art (priority field 3).

On real SNES hardware, sprite-vs-sprite priority is decided purely by OAM index — the lowest-numbered object wins each pixel. The OAM priority field only chooses where that pixel sits relative to background layers, not relative to other sprites.

The renderer instead treated each OBJ priority level as a fully independent layer (draw all priority-0 sprites, then priority-1, …, each pass overwriting the last). So a high-index priority-3 sprite would overwrite a low-index priority-2 sprite — exactly the cursor bug.

Fix

Resolve the whole OBJ layer once per scanline by OAM index into per-line buffers (_obj_line_color / _obj_line_pri / _obj_line_layer), with the lowest-index sprite winning each pixel along with its priority bit. Each draw_objects(priority=N) pass then blits only the pixels whose owning sprite has that priority. The mode dispatcher's per-priority calls and BG interleaving are unchanged, so BG/OBJ layering is preserved. This also fixes same-priority sprite ordering (lowest index on top).

Tests

Added TestSpriteVsSpritePriority covering the cross-priority OAM-index rule (two overlapping sprites). All three new tests fail on the pre-fix code and pass with the fix.

Verified against the save state: the bomberman-head cursor now renders in front of the character, left of "NORMAL GAME". Full pysnes/ppu/ suite passes except two pre-existing Mesen-oracle BG failures (bg_4bpp, tile_flip) that also fail on main, unrelated to sprites.