* perf(git): read both diff blobs concurrently
The diff loaders awaited their two sides in series, so the second `git show`
could not start until the first had returned. The reads are independent, so
that was pure added latency on every diff the review panel opens: ~47 ms
sequential vs ~24 ms concurrent, a saving of ~23 ms per diff.
Covers the merge-base, commit, and staged loaders, plus the unstaged path where
the working-tree read is independent of the index->HEAD chain. The unstaged
left chain itself stays sequential because its second step depends on the first.
The staged coalescing test asserted the sequential shape (one spawn, then the
next); it now pins the contract that actually matters — eight identical reads
still collapse to two spawns, one per side.
* test(perf): interleave the diff-blob benchmark arms
Running one strategy's whole batch before the other's lets cache warming, CPU
frequency drift, and background load correlate with the strategy being measured.
Alternate the arms per iteration, alternate which goes first, and report medians
so that drift stays common to both.
Also reject malformed env settings rather than truncating them — Number.parseInt
accepts "10foo" and 3.5.
Interleaved result confirms the original: 1.90x-2.03x, ~24 ms saved per diff.
Neil