bug: durable: WithMaxEventDataBytes counts JSON bytes, but FileStore stores base64, so a passing payload is refused after the kernel moved #242

Module

durable

What happened

• The fix(durable): journal WithEventData payloads for recovery #230 size check counts the event data's JSON bytes before the kernel runs.
• FileStore stores the bytes base64-encoded (about 4/3 larger) in one journal line, capped at 16 MiB.
• With a raised WithMaxEventDataBytes, a payload under the limit passes the check, the kernel transitions, then Append fails with ErrRecordTooLarge.
• The live instance has moved, and the store has not.
• The 8 MiB default is safe: 8 MiB base64-encodes to about 10.7 MiB.

Expected behavior

• A payload that passes the size check is one the store can write.
• Or: WithMaxEventDataBytes refuses a limit the store can't hold, or the check counts the stored size.

Reproduction

On main @ 494c78a, in package durable_test (uses the existing linearMachine, runCtx helpers):

func TestFileStore_EventDataUnderLimit_IsStored(t *testing.T) {
	ctx := context.Background()
	st, _ := durable.NewFileStore(t.TempDir())
	r := durable.NewRunner(linearMachine(), st,
		durable.WithMaxEventDataBytes[string, string, *runCtx](14<<20))
	h, _ := r.Start(ctx, "big", &runCtx{}, state.WithInitialState("idle"))
	payload := strings.Repeat("a", 13<<20) // 13 MiB JSON, about 17.3 MiB base64
	if _, err := h.Fire(ctx, "go", state.WithEventData(payload)); err != nil {
		// fails: ErrRecordTooLarge, and Current has already moved
		t.Fatalf("refused after the kernel ran (current=%q): %v",
			h.Instance().Snapshot().Current, err)
	}
}

Environment

• Crucible module version: main @ 494c78a
• Go version (go version): go1.27.3
• OS / arch: darwin/arm64