fix(tree-sitter): simplify arco-kdl multiline strings

[pesap]

Summary

This PR isolates the tree-sitter overlay cleanup on top of main.

It makes three focused parser changes:

1. Replace the custom multiline-string scanner path with grammar-level triple-quoted string parsing.
2. Simplify the overlay grammar by factoring shared node and math-body helpers.
3. Allow quoted string literals inside algebra text, with corpus and fixture coverage.

Scope

• tree-sitter overlay only
• no runtime or compiler changes

Files touched

• tools/tree-sitter-arco-kdl/grammar.js
• tools/tree-sitter-arco-kdl/src/grammar.json
• tools/tree-sitter-arco-kdl/src/node-types.json
• tools/tree-sitter-arco-kdl/src/parser.c
• tools/tree-sitter-arco-kdl/src/scanner.c
• tools/tree-sitter-arco-kdl/test/corpus/arco_math.txt
• tools/tree-sitter-arco-kdl/examples/highlight_demo.kdl

Validation

• cd tools/tree-sitter-arco-kdl && tree-sitter test
• just kdl-overlay-check