feat(launcher): cache manifest.launcher.md per module on init

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>

manifest_loader.lua

@@ -0,0 +1,31 @@
+local M = {}
+
+local fm
+
+local function ensure_deps()
+    if not fm then
+        fm = dofile(engine.module.dir_of("lib-management.launcher") .. "/frontmatter.lua")
+    end
+end
+
+-- Returns:
+--   meta = { summary, description_md, author, tags, teaser_images,
+--            carousel_interval_seconds, ... }
+--   body_blocks = output of markdown.parse(body) — pre-parsed for cache reuse
+-- On any failure (file missing, parse error): meta = {}, body_blocks = {}.
+function M.load_for(module_id)
+    ensure_deps()
+    local content, err = engine.module.read_file(module_id, "manifest.launcher.md")
+    if not content then
+        return { _missing = true, _err = err }, {}
+    end
+    local meta, body, perr = fm.parse(content)
+    if not meta then
+        return { _missing = false, _err = perr }, {}
+    end
+    local md = dofile(engine.module.dir_of("lib-management.launcher") .. "/markdown.lua")
+    local blocks = md.parse(body)
+    return meta, blocks
+end
+
+return M