fix: invalidate stale raw spelling on combinator value assignment

[SAY-5]

Fixes #196.

After parsing a\tb, assigning combinator.value = ">" still serializes the old tab because raws.value wins over the new value. Clear that stale raw spelling on explicit assignment, while preserving constructor-provided raw text, unchanged clones, and the existing raw-property helpers. Assigning the same semantic space also normalizes a tab or newline as requested in the issue.

Document that replacing the value replaces comments embedded in raws.value; comments stored separately in raw spacing remain intact. Add regression tests for whitespace, named combinators, constructor option order, clone isolation, and raw helper behavior.

Validation on Node 26.3.0: seven regression assertions fail before the fix; all 75 focused tests and all 813 full-suite tests pass afterward. Coverage gates pass (94.96% lines, 95.50% branches, 97.72% functions). Build, lint, changed-source formatting, and strict declaration tests with project-local type roots pass.

The default typecheck picks up unrelated ancestor @types/node declarations and reports three errors there; the strict check with --typeRoots ./node_modules/@types passes without skipping library checks. Full formatting reports only the unchanged src/__tests__/exceptions.mjs; lint has two existing warnings in that file.

OpenAI Codex assisted with the code, tests, and description. The final patch received an independent AI review.