Fix climber jump sprite disappearing (#276)

[jonathanpeppers]

Problem

The player sprite disappeared during jumps in the climber sample (#276).

Root Cause

The transpiler's HandleStelemI1 in IL2NESWriter.ArrayHandling.cs only tracked ONE standalone value local when analyzing the value expression for array stores. For the pattern:

byte prev_lo = actor_yy_lo[pi];
actor_yy_lo[pi] = (byte)(prev_lo + dv);

The second ldloc (dv) overwrote the first (prev_lo) in valueLocalIdx, and addValue stayed 0 because the add instruction wasn't preceded by a constant. This caused the transpiler to emit:

LDA dv        ; loads dv
CLC
ADC #$00      ; adds 0 instead of prev_lo!
STA arr,X

Instead of the correct:

LDA prev_lo   ; loads first local
CLC
ADC dv         ; adds second local
STA arr,X

This made actor_yy_lo[pi] = dv instead of prev_lo + dv, causing the Y position to diverge wildly from the scroll position. After a few frames, rel_hi != 0 triggered the off-screen check, hiding the player sprite.

Fix

Track both value locals (valueLocalIdx + valueLocalIdx2) in the stelem handler's backward IL scan. When both are present with an add or sub operation, emit LDA local1; CLC; ADC local2 (or SEC; SBC local2).

Verification

• All 545 tests pass (481 dotnes.tests + 64 analyzer tests)
• Climber verified.bin updated to match the corrected output
• Emulator debug confirms correct jump physics: Y increments by 4, 4, 4, 3, 3... (vel>>2) instead of being set to the raw dv value
• Player sprite (tiles $E8-$EB) remains visible throughout the jump animation

[Copilot]

Pull request overview

Fixes a transpiler codegen bug that caused incorrect byte-array element updates in the climber sample, which in turn made the player sprite disappear during jumps.

Changes:

• Extend HandleStelemI1 value-expression analysis to track a second standalone ldloc operand.
• Emit LDA local1; CLC/SEC; ADC/SBC local2 for arr[i] = (byte)(local1 +/- local2) patterns.
• Update the climber verified.bin snapshot to reflect corrected ROM output.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/dotnes.tasks/Utilities/IL2NESWriter.ArrayHandling.cs	Updates stelem.i1 pattern detection/emission to support two-local add/sub value expressions.
src/dotnes.tests/TranspilerTests.Write.climber.verified.bin	Snapshot update for climber ROM bytes after corrected codegen.