refactor(launcher): memoize markdown.lua in manifest_loader ensure_deps

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.lua

@@ -1,11 +1,14 @@
 local M = {}
 
-local fm
+local fm, md
 
 local function ensure_deps()
     if not fm then
         fm = dofile(engine.module.dir_of("lib-management.launcher") .. "/frontmatter.lua")
     end
+    if not md then
+        md = dofile(engine.module.dir_of("lib-management.launcher") .. "/markdown.lua")
+    end
 end
 
 -- Returns:
@@ -23,7 +26,6 @@ function M.load_for(module_id)
     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