test(api): measure the registration write bound in an isolated process

[DecisionNerd]

Closes #1941.

Why

registration_copies_and_writes_no_source_bytes (from #1923) asserts that registering a 641 KB source writes under 64 KiB. The count comes from RegionCapture::written_bytes, which is wchar from /proc/self/io and covers every thread in the process (crates/graphforge-storage/src/concurrency_attribution/capture.rs:436,453). Nextest isolates tests in separate processes, but the lean lane runs cargo test -p graphforge-api --no-default-features (.github/workflows/test.yml:285), where other tests write concurrently. It failed on #1940, a YAML-only PR, with registration wrote 307842 bytes (run 37876583854).

Change

Each route now runs registration_write_bound_child alone in a fresh process. This is the same env-gated child pattern construction_clock_tests.rs uses. The parent asserts that the child succeeded and printed 1 passed, so a filter mismatch can't pass vacuously. The < 64 KiB bound and every other assertion are unchanged; nothing is skipped, retried or relaxed.

Validation

• cargo test --locked -p graphforge-api --no-default-features --lib, the lane's configuration with the full suite running concurrently on a loaded host: 433 passed, 0 failed.
• Known positive: with the bound temporarily set to 1 byte, the isolated child reports registration wrote 1072 bytes and the parent fails. In isolation the registration writes about 1 KiB, against the 307,842 bytes measured while sharing a process. Reverted before commit.
• cargo nextest run -p graphforge-api --no-default-features --lib for both tests: 2 passed.
• cargo fmt --check passes. Clippy doesn't lint test code in make check.

🤖 Generated with Claude Code

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