Store oversized step results using mutation file storage - #310

Preserve oversized queued query, mutation, and action step results in file storage before recording the step failure. The blob, oversized-value record, and failed step commit together, so preservation cannot race with workflow cleanup. Results still fail above the existing 800 KiB threshold; the error includes the step ID and a mutation call for retrieving the full value.

Create the blob and compare Blob.size with ctx.meta.getTransactionMetrics().fileWriteBytes.remaining, also checking filesWritten.remaining. These are server-reported budgets, with no hardcoded storage limits. If the value cannot fit, record a terminal step failure explaining that it was not preserved. There is no scheduled preservation or pending state, and unrelated storage errors propagate.

Oversized events retain their existing failure-and-preview behavior without file preservation, whether sent before or after the workflow starts waiting. Inline results, workflow final returns, and oversized arguments remain outside this feature. Add paginated list and mutation read/remove APIs, cleanup of stored files, and README documentation.

This PR is standalone on main and incorporates the storage work from #242. Requires released Convex 1.48 (convex@1.48.0 in development, ^1.48.0 as a peer). Tests use released convex-test@0.0.67 with native mutation-storage metrics and limit enforcement. Budget tests configure actual transaction limits, without metrics mocks or storage syscall adapters. Live probes also validate backend accounting and limits.

Validation:

• All 145 tests, build, and TypeScript checks pass. Lint has no errors and three existing warnings.
• Unit coverage includes exact byte-budget boundaries, UTF-8 and Convex-value round trips, exhausted byte/file budgets with no writes or scheduling, unrelated storage errors, 11 oversized pre-sent events failing without preservation, workflow cleanup and restart deleting preserved records and blobs, and removal rollback preserving a readable value.
• Manual development-deployment checks verified 900 KB result retrieval, removal rollback, idempotent removal, cleanup, and both event timings without preservation. A 3 MB result that serializes to an 18 MB blob fails cleanly through the real queued completion path.
• Manual backend validation confirmed that backend file-byte usage equals Blob.size. It also verified a storage-limit rejection can be caught without consuming storage budget or preventing a smaller write from committing. HTTP downloads return 200; test storage is empty after cleanup.