feat: add anchor placement validation

[zharinov]

Summary

• Add AnchorInAlternation diagnostic to reject anchors directly in alternations (e.g., [(a) . (b)])
• Parser now emits helpful error with fix suggestion: use [{(a) . (b)} (c)] to anchor within a branch
• Update lang-reference.md and CLAUDE.md with anchor placement rules

Test plan

• make test passes (675 tests)
• New tests in anchors_tests.rs verify error messages