Add parser conformance matrix

[stacknil]

Summary

• add docs/parser-conformance-matrix.md
• document supported input formats, normalized events, unsupported buckets, and structural warning buckets
• link each matrix section to the current parser code and fixture corpus expectations

Validation

• git diff --cached --check passed locally before the remote connector commit
• touched-file privacy scan for local paths, identities, and secret-like strings found no issues
• referenced parser source, tests, and fixture paths were checked locally

Note: this is docs-only; no C++ behavior changed.