Closes #1011

Summary

Replaces per-line full-history reduce scans and .slice().reverse().find(...) searches in parseCodeBlocks (packages/editor/packages/editor-core/packages/editor-state/src/features/code-blocks/features/blockHighlights/codeParser.ts) with incrementally maintained per-parse lookup state:

• Tracks openBlockCount directly (+1 on block open, -1 on closing match, 0 net change on else transition) instead of reducing over all previously discovered blocks on every source line.
• Maintains a Map<string, CodeBlock[]> of open-block stacks keyed by endInstruction (openBlocksByEndInstruction) and a separate openIfBlocks stack for open if blocks so closing instructions and else locate their nearest open target in $O(1)$ amortized time without copying or reversing blocks.
• Preserves tolerant highlight parsing for incomplete blocks, repeated else instructions, inner if ... ifEnd blocks nested inside an outer if ... else ... ifEnd, stray closers, and mismatched nesting (if ... loop ... ifEnd ... loopEnd).
• Adds unit tests in codeParser.test.ts covering inner if blocks closed before an outer else, repeated else instructions, and closing instructions matched behind differently typed open blocks.

Benchmark Results

Measured parseCodeBlocks on blocks containing module + $N$ sibling if / else / loop / loopEnd / ifEnd constructs (median of 11 alternating samples after 4 warmups on Node v24.16.0):

Verification

• biome check on codeParser.ts and codeParser.test.ts
• tsc --noEmit on @8f4e/editor-state
• vitest run on codeParser.test.ts and updateGraphicData.test.ts (15 passed)