[Pro npm] Small residuals: ENOENT loadable-stats misconfig permanently silent; add success-cache regression test; worker.test.ts bypasses formAutoContent shim

[justin808]

Summary

Three small Pro-npm residuals from the parser/loadable-stats fix wave (#4401/#4447) and the multipart consolidation (#4435).

Findings

1. ENOENT loadable-stats misconfig is permanently silent

warnIfUnexpectedLoadableStatsFailure (packages/react-on-rails-pro/src/injectRSCPayload.ts:163-164) returns early on ENOENT, so a setup that copies loadable-stats.json to the wrong directory never logs anything, ever — the exact misconfiguration issue #4371's impact section wanted a single log line to surface. #4401/#4447 fixed the retry and rate-limited non-ENOENT warnings, but ENOENT gets zero notes. Fix: one info/debug-level note per process on the first ENOENT (path included), silent thereafter, preserving the opportunistic no-stats fallback.

2. Missing success-cache regression test

tests/loadClientChunkStylesheetHrefs.test.ts covers fail→retry→success but never renders again after success to assert readFileSync is not called a third time. The success-permanent-cache behavior (injectRSCPayload.ts:266-268, 319-322) is the no-per-request-re-read guarantee from #4371; a one-assertion extension of the existing monotonic-clock test locks it in against future refactors of the load-state machine.

3. worker.test.ts bypasses the formAutoContent shim

PR #4285 added a direct import FormData from 'form-data' + manual form.append(...)/getBuffer()/getHeaders() block to packages/react-on-rails-pro-node-renderer/tests/worker.test.ts:17, 135-168 (unsafe-filename rejection test), while the same file imports the ./formAutoContent shim that #4435 (issue #4407) standardized the inject-transport tests on. The shim already supports {value, options} pass-through incl. filepath/contentType. Migrate the new test to the shim (or extend the shim's return with the raw form for buffer access) so worker.test.ts uses one multipart pattern again. Test-only, cosmetic.

Acceptance criteria

• First-ENOENT emits one diagnostic note per process; still silent thereafter.
• A test asserts no re-read after a successful loadable-stats read.
• worker.test.ts uses the formAutoContent shim (no direct form-data import).

Audit ledger: #4010 (comment)

Post-merge audit follow-up (fix-wave a6c5867..ed5049f). Filed by Claude Code on behalf of Justin Gordon.