build: add schema, validator and listing policy

[datlechin]

• Schemas: entry manifest (closed, name at most 32 characters), curation files, and the published index (tolerant reader; a release holds index.json, SHA256SUMS and each PNG as a flat, content-hashed asset).
• Curation: TableProApp as the only Official publisher, no partners, reserved slugs and names, hashes of the TablePro icon.
• scripts/validate.py and registry.py: schema, text, naming, tier, dates, images and icon resemblance, change scope, online GitHub id, license, ownership and install checks, private-file source scan.
• 103 unit tests on fixtures. They pass with an empty integrations/.
• validate.yml with the Registry Gate job on every pull request, triage.yml (labels only, no checkout), stale.yml. Actions pinned by SHA, actionlint checked by checksum, zizmor in CI.
• requirements.txt from pip-compile --generate-hashes, installed with --require-hashes.
• POLICY.md v1, CONTRIBUTING, SECURITY, MIT LICENSE, LICENSE-DATA (CC0 1.0 legal code plus the icon and trademark carve-out).
• CODEOWNERS (@TableProApp/maintainers), pull request template, issue forms, Dependabot.
• _owner/setup.sh and the two rulesets, for the owner to run after merge.