SCSS variable extraction uses wrong node type #9

Problem

SCSS `$variable: value;` declarations are not extracted as variable nodes. The language spec likely maps the wrong tree-sitter node type.

Root Cause

SCSS variables use `$name: value` syntax which produces a specific tree-sitter node type that doesn't match what's configured in the language spec.

Fix

1. Run AST dump on a SCSS file to find the correct node type for `$variable` declarations
2. Update `VariableNodeTypes` in `internal/lang/scss.go`
3. Verify with Bootstrap: https://github.com/twbs/bootstrap

Impact

Fixes variable extraction for SCSS projects. Small, self-contained fix.

References

• Benchmark report: `BENCHMARK_REPORT.md` (SCSS section)
• Language spec: `internal/lang/scss.go`