fix: lint generated grammar by default

[sit-d]

Summary

• fix scripts/lint_grammar.py so its no-argument default lints the generated grammar in dist/fasmg.tmLanguage.json
• document that the standalone linter now matches the path used by scripts/gen_all.py --lint

Verification

• python3 -m py_compile scripts/*.py
• python3 scripts/lint_grammar.py → lint: all 46 regex patterns OK in fasmg.tmLanguage.json
• python3 scripts/gen_all.py --lint → generated both dist outputs and reported lint: all 46 regex patterns OK in fasmg.tmLanguage.json
• git diff --check

Notes / Limitations

• scripts/gen_all.py --lint rewrote generated dist files in the working tree during verification; those generated-output changes were reverted because the only intended fix here is the linter default path, and the existing open docs PR already touches the generated UDL drift.
• Full scripts/validate.py fixture assembly was not run because this Linux cron environment does not have fasmg installed.

[sit-d]

Added fresh ad-hoc verification after the final code change:

• Temporary verifier: /tmp/hermes-verify-qulck70h.py (removed after run)
• It asserted there is no root-level fasmg.tmLanguage.json, dist/fasmg.tmLanguage.json exists, no-arg python3 scripts/lint_grammar.py exits 0, explicit python3 scripts/lint_grammar.py dist/fasmg.tmLanguage.json exits 0 with matching output, and git diff --check exits 0.
• Result: ad-hoc verification passed: no-arg lint uses dist/fasmg.tmLanguage.json and matches explicit dist lint.