Make in-memory fs growth allocation-fallible

InMem writes and extending truncates currently rely on infallible Vec/Cow growth, which may panic or abort on capacity or allocation failure. Define failure atomicity and the appropriate fs error, then use fallible reservation before mutation. Deferred from #1318 to keep allocation behavior unchanged during the broker refactor.

The issue identifies InMem writes and extending truncates as entry points; start by tracing those paths and reviewing the allocation behavior context from #1318. Establish failure atomicity and the appropriate filesystem error, then verify that allocation failure leaves state unchanged.

Written by the indexing model from the issue text.