perf(sync): raise the default fetch ceiling floor to 50 threads

[srnnkls]

What

When --jobs is unset, the default rayon pool size becomes min(units, max(2×cores, 50)) instead of min(units, 2×cores). --jobs still overrides. Also fixes the benches/fetch_sweep.sh reporter, which BSD awk (macOS) rejected outright.

Why

benches/fetch_sweep.sh (re-run 2026-07-10, 8-core machine; consistent with the 2026-06 baseline in the async-fetch scope findings) shows fetch is network-wait-bound and the pool ceiling — not the box — is the limit for many small sources:

20 tiny git sources, cold	real	cpu	io_wait
jobs=1	14.49s	0.61s	96%
jobs=16 (old default cap)	1.54s	0.68s	56%
jobs=50	0.95s	0.76s	20%

Aggregate CPU stays flat as jobs rise (heavy fixture: 13.96s → 12.59s across jobs 8→32), so the wider ceiling costs parked-thread memory, not cycles. The floor of 50 captures the measured win; boxes where 2×cores already exceeds 50 keep their existing ceiling.

Considered and rejected

Separating the fetch pool from the compute pool: the same sweep shows the machine is never CPU-saturated during sync (~0.44 of 8 cores busy on 11 medium/large repos) and there is no oversubscription penalty, so a dedicated compute pool has nothing to relieve.

Verification

• cargo test: 1349 lib + all integration suites green
• cargo clippy --all-targets -- -D warnings: clean
• benches/fetch_sweep.sh runs end-to-end again (reporter fix) — numbers above are from this branch's harness

[Copilot]

Pull request overview

This PR increases the default Rayon thread-pool size used by sync when --jobs is not provided, raising the pool cap to at least 50 threads to better overlap network waits during fetch-heavy sync workloads. It also fixes the benchmark sweep script’s awk reporter to be compatible with BSD awk (macOS).

Changes:

• Raise default pool cap from 2×cores to max(2×cores, 50) (still bounded by units and still overridden by --jobs).
• Update/expand unit tests to cover the new cap-floor behavior.
• Fix benches/fetch_sweep.sh awk logic for BSD awk portability.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/sync/resolve.rs	Adjusts default sync Rayon thread count cap (adds a minimum cap of 50) and updates tests/docs accordingly.
benches/fetch_sweep.sh	Makes the benchmark output formatter compatible with BSD awk by avoiding the prior inline conditional expression.

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