PERF.4 Apps: modulepreload hints for the static import closure, so the import waterfall is one round trip deep instead of 4 to 8

Measured 2026-11-08 (static import graph from each release's entry module): cadcraft 281 modules, depth 6 (99 at depth 1, 104, 51, 21, 4, 1), 4.6 MB; bimcraft 422 modules, depth 4, 6.5 MB; partcraft 269 modules, depth 8; markupcraft 78 modules, depth 3. The browser only discovers level n+1 after level n arrives, so each level costs a round trip even with HTTP/2 (PERF.1).

Plan: AppBuilds::headTags adds <link rel="modulepreload" href=...> for the entry's static closure (regex over import/export-from lines, cached per release in storage/ and per lab stamp for a Discord member builds; about 25 KB of tags for cadcraft, gzips well). Dynamic import() sites (38 in cadcraft, 97 in bimcraft) stay lazy. Measure with tools/perf/startup.mjs before and after; most valuable after PERF.1, still a win on HTTP/1.1 for the small deep levels.