Publish Mantis Extension page

[yrstm]

What changed

• Add a dedicated Mantis Extension page at /mantis/extension/.
• Link the extension from the public Mantis navigation, hero, product callout, README, and changelog.
• Describe the Chrome and Safari capture flow, privacy boundary, limitations, and honest pre-release availability.
• Record that the third-party fallback and host permission were removed; page URLs and content stay local to the extension capture path.
• Keep Chrome Web Store and signed Safari App Store distribution explicitly deferred.
• Replace the unpublished npm-registry command with an immutable GitHub archive install.
• Pin the public converter to core commit 9e462696990449979b309886d3581dd3201c2b17 with matching SHA-384 integrity.
• Add repository tests for the canonical URL, internal anchors, availability claims, shared dependency pin, and SRI.

Share URL

https://yrstm.github.io/mantis/extension/

Validation

• npm test: 72 passing.
• npm run types:check: passing.
• npm run benchmark: 129/129.
• npm run perf: fidelity green.
• npm run pack:check: passing with an isolated writable cache.
• npm run test:browser: all Chromium assertions passing.
• Desktop QA at 1440×1000 and mobile QA at 390×844: no overflow or missing anchors.
• Main-page HTML-to-Markdown converter exercised successfully in the browser.
• Browser console and page-error logs: clean.
• Pinned jsDelivr file is byte-identical to the reviewed local mantis.js.