Fix scroll tracking during falls in climber

[jonathanpeppers]

Fixes #318

Changes

• Player sprite (actor 0) is never hidden by the off-screen check. When rel_hi != 0, the sprite is clamped to Y=224 instead of being skipped. This matches the original climber.c which uses signed 16-bit math for screen_y and never culls the player.

• player_screen_y is always updated for actor 0, even when off-screen, so the scroll handler can track the player during falls.

Known Limitation

On certain randomly-generated floor layouts, the byte-pair subtraction for rel_hi can produce incorrect results when yy and scroll_yy have certain page-boundary relationships. This causes the player to appear at the wrong screen position temporarily during falls on high floors. A proper fix requires the transpiler to support signed 16-bit subtraction for the screen_y computation, matching the original C code's int arithmetic.

Mesen traces confirm the fall physics itself is correct at all floor heights (4, 6, 8, 10, 12).

[Copilot]

Pull request overview

This PR updates the climber sample to prevent the player sprite from being culled during high-floor falls when scroll tracking temporarily lags behind the player, addressing issue #318 (visual disappearance due to off-screen logic).

Changes:

• Adjust sprite rendering logic so actor 0 (player) is never culled by the rel_hi != 0 off-screen check; instead, the player’s screen_y is clamped to 224 when off-screen.
• Ensure player_screen_y is updated for the player even when clamped, so the scroll handler can continue tracking during falls.
• Update the test fixtures (compiled DLLs + verified ROM snapshot) to match the new sample behavior.

Reviewed changes

Copilot reviewed 1 out of 4 changed files in this pull request and generated 1 comment.

File	Description
samples/climber/Program.cs	Changes sprite culling/clamping logic for the player and updates player_screen_y to keep scroll tracking correct during falls.
src/dotnes.tests/Data/climber.debug.dll	Updated compiled debug fixture for the climber sample used by snapshot tests.
src/dotnes.tests/Data/climber.release.dll	Updated compiled release fixture for the climber sample used by snapshot tests.
src/dotnes.tests/TranspilerTests.Write.climber.verified.bin	Updated verified ROM snapshot to reflect the new climber sample output.

[jonathanpeppers]

🤖 Resolved merge conflicts with main (binary test data files — rebuilt DLLs and snapshot).

All 611 tests pass. cc65 stack stable at $0800 through 6000 frames.

Code review:

The fix is correct — the player (actor 0) should never be culled from the sprite draw loop because player_screen_y must always be updated for scroll tracking. The original C code uses signed 16-bit math that naturally handles this; the byte-pair arithmetic in dotnes loses the sign, so clamping to 224 is a reasonable workaround.

One concern from the PR description: "On certain randomly-generated floor layouts, the byte-pair subtraction for rel_hi can produce incorrect results." This is a pre-existing limitation of the byte-pair scroll math, not introduced by this PR.

Ready for review.