fix(disk buffer): backpressure on runtime capacity errors

[Jansen-w]

Summary

Runtime filesystem capacity errors can be temporary, but disk-buffer writes previously treated them as fatal I/O failures. This change makes disk_v2 writers:

• recognize StorageFull, ENOSPC, and EDQUOT errors;
• retain partially written bytes and resume from the committed offset without duplication;
• apply backpressure within the owning write or flush future while waiting for reader progress or exponential retry;
• preserve logical when_full behavior while physical capacity exhaustion consistently backpressures every policy;
• emit one structured capacity-exhausted event and one recovery event per episode, with buffer path, operation, retry count, delay, and duration;
• preserve structured fatal synchronization errors and keep startup/non-capacity failures fatal.

Production writes remain session-owned synchronous syscalls. A kernel write cannot be portably cancelled and can delay cancellation until it returns; keeping it session-owned prevents writes from outliving the buffer lock or racing a reopened buffer.

Vector configuration

No new configuration is required. The behavior applies to existing disk_v2 buffers.

How did you test this PR?

• cargo test -p vector-buffers (132 passed, 2 pre-existing ignored)
• cargo clippy -p vector-buffers --all-targets -- -D warnings
• cargo fmt --all -- --check
• cargo vdev check events
• cargo vdev check changelog-fragments
• Deterministic regressions for partial writes, cancelled flush/write resume, small and large writes after cancellation, timer/reader wakeups, retry ownership and telemetry, atomic/fallback file opens, rotation synchronization, startup failures, and non-capacity errors

Real exhausted-filesystem and Windows-specific behavior remain covered through deterministic fault injection rather than platform integration tests.

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. A changelog fragment is included.
• No. A maintainer will apply the no-changelog label to this PR.

References

None.