Run the Bun dev child with the transpiler cache off

[vivek7405]

Bun caches the transpile of any source over 50KB on disk. The cache key is the file's content, and the cached output keeps the absolute paths that a Bun.plugin onResolve returned. The dev alias resolver (#1575) returns absolute paths, so the cached output of a large app module was tied to the checkout that ran webjs dev first. Any other copy of the file (a worktree, a moved app, a later webjs start) then imported from that old directory. When the old directory was gone, the result was a 500. When it still existed, the page silently ran the other copy's code. This is why website / returned 500 on Bun in every local worktree, which failed test/bun/app-boot.mjs and site-seo-tags.

The supervisor now starts the Bun child with BUN_RUNTIME_TRANSPILER_CACHE_PATH=0. Bun reads that setting only at startup, which is why it has to be set on the child. New Bun proof: test/bun/dev-transpiler-cache.mjs fails when the fix is reverted. It is wired into ci.yml and webjs.ci.

https://claude.ai/code/session_01SZ72LSPAo4NvYvBDSD6RLo

[pilots-run]

Preview for 2483fd8 was not built: compose file has unsupported keys

Next: fix the listed keys in the compose file

[vivek7405]

Its commit landed in main through #1600 (cherry-picked unchanged), so this PR is now redundant.