docs: add fasmg syntax project overview

[sit-d]

Summary

• add a root README.md so the repository landing page explains the fasmg syntax-generation project
• map the main script, generated output, fixture, and Tree-sitter paths to their purposes
• document quick local checks and the optional assembler-backed validation command without overlapping the existing validation-workflow PR

Verification

• python3 -m py_compile scripts/*.py
• python3 scripts/lint_grammar.py dist/fasmg.tmLanguage.json → lint: all 46 regex patterns OK in fasmg.tmLanguage.json
• ad-hoc verifier /tmp/hermes-verify-fasmg-readme-f8b4drpl.py checked README path references, documented commands, ran the explicit grammar linter, and ran git diff --check; it exited 0 and was removed

Notes / Limitations

• python3 scripts/gen_all.py --lint was also exercised during inspection and reported all 46 regex patterns OK, but it rewrote the existing generated UDL drift; that generated-file diff was intentionally reverted because PR docs: document fasmg syntax validation workflow #1 already covers generated UDL/documentation drift.
• Full scripts/validate.py --fasmg ... fixture assembly was not run because this Linux cron environment does not have a fasmg executable installed.
• Related open PRs at inspection time: docs: document fasmg syntax validation workflow #1 documents the test-suite validation workflow; fix: lint generated grammar by default #2 fixes the no-argument linter default. This README uses the explicit dist/fasmg.tmLanguage.json linter path to avoid depending on fix: lint generated grammar by default #2.