Revert "perf(blob): one syscall per new chunk in write_blob_bytes"

This reverts ee51b32. The create_new change showed no measurable throughput
benefit — three 9-rep interleaved runs on the same ext4 device swung −12%..+21%
at the 256 KiB CDC size (a negative stat on a warm dentry cache is ~µs, below
the shared-disk noise floor). Applying the same "no change without a measured
win" bar used for the pool-sizing revert: the idiom/TOCTOU angle is real but the
race is already prevented upstream by the PG pin-or-classify serialisation, so
it's defence-in-depth only — not enough to keep an unmeasured change. Reverts
the production change, the bench, and its doc together.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JG5yYZ9s868mJwqT2Qz7ez

Cargo.toml

@@ -199,14 +199,6 @@ name = "bench_pool_concurrency"
 path = "examples/bench_pool_concurrency.rs"
 required-features = ["bench"]
 
-# Blob write syscall benchmark — per-chunk stat removal in write_blob_bytes
-# (try_exists+create → create_new). Measures new-chunk write throughput, old
-# vs new strategy, swept over chunk size. No Postgres needed.
-[[example]]
-name = "bench_blob_write"
-path = "examples/bench_blob_write.rs"
-required-features = ["bench"]
-
 # ACL owner-cache benchmark — owner query vs moka hit (needs the dev Postgres up).
 [[example]]
 name = "bench_owner_cache"