A repository's README is rendered once per blob: the files page's readme and render under 15 ms warm #6148

Measured on 2026-10-26: /trevato/forge/files spends 10–12 ms in readme and 34 ms in render, and /trevato/forge/blob/main/README.md renders in 29 ms. trevato/forge's README is 205 KB, and its HTML is rebuilt from the markdown on every load.

Done when:

• [ ] readmeFor (src/web/pages/repo.ts:434), or the markdown step it calls, caches the rendered HTML by blob oid, in memory and bounded by bytes.
• [ ] A test asserts that the second render of the same blob does no markdown work, that a changed README renders anew, and that the output is byte-identical to the uncached render.

Files: src/web/pages/repo.ts (readmeFor) or src/web/markdown.ts, test/readme-cache.test.ts. Risk: the rendered HTML must stay escaped exactly as today. The test must include a README containing <script>.