Fix buffer stage size accounting race

[simonyang08]

Which issue(s) this PR fixes:
Fixes #5479

What this PR does / why we need it:

Buffer#write previously deferred the staged-byte metric update until after releasing every chunk lock. A concurrent enqueue_chunk could remove the staged chunk and subtract its committed size before the matching addition, making the raw stage_size gauge transiently negative.

This change records committed staged bytes before releasing each chunk monitor. It deliberately does not acquire the Buffer-global lock in that position, preserving the existing lock-order constraint. A deterministic regression test pauses the writer at mon_exit and verifies that the staged bytes transfer to the queue without a negative intermediate gauge.

Docs Changes:

None.

Release Note:

Fix a race that could make the internal buffer stage_size gauge transiently negative while a write and chunk enqueue overlap.

Validation:

• bundle exec ruby -Itest test/plugin/test_stage_size_race.rb - 1 test, 5 assertions, passed
• repeated regression test 10 times - passed
• bundle exec ruby -Itest test/plugin/test_buffer.rb - 73 tests, 431 assertions, passed
• Ruby syntax checks for both changed files - passed
• git diff --check - passed
• bundle exec rake test was also attempted under WSL on /mnt/d; unrelated environment failures were observed around DrvFS permissions, CRLF shebangs, Unix sockets, and unavailable YJIT. The focused buffer suites above are green.

AI assistance:

This change was prepared with OpenAI Codex assistance and reviewed and validated by the contributor.