Matches the existing frontmatter.lua memoization pattern. Avoids
re-dofile'ing markdown.lua per successful load_for call (matters
once L.9 rolls out manifest.launcher.md to every module and the
init-time call count goes from 1 to N).
manifest_loader.load_for(id) reads <install>/modules/<id>/
manifest.launcher.md via engine.module.read_file, runs the JSON
frontmatter through frontmatter.parse, and pre-parses the body via
markdown.parse. Cached on the shared ctx so the detail-panel render
loop does not pay the parse cost per frame. Missing files degrade to
{_missing=true} and the list falls back to the module's name/id.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
