29. Shifting Reused Subtrees into the Parser's Chunks

• After an edit, a memo result after the edit is reused with its positions shifted: shiftNode copies its node tree. Every copied node, child list and field list was a separate heap allocation, and every copy made a new map of the nodes already copied (to keep shared subtrees shared). In the incremental benchmark this was 89% of the bytes allocated per edit and reparse.
• Copies now come from the parser's chunks (newNode, nodes, fields), and the map is kept by the parser and cleared between copies (and dropped when it grew past 1,024 entries, so clearing stays cheap).
• Effect (min of 8 interleaved runs, Apple M3 Max, one-character edit to minilang plus reparse): 1.72 → 1.20 ms (closure), 1.76 → 1.20 ms (bytecode), 1.77 → 1.25 ms (iterative); 20k → 0.15k allocations, 3.0 → 2.4 MB.