[abhinav]

Loading tracked branches runs individual state and head lookups in a
worker pool, starting Git processes for every branch.
Load their state through Store.LookupBranches and GetMany, and
resolve requested local heads in one filtered for-each-ref call.
The worker pool is no longer needed to overlap state and head lookups.

Expose ordered Service.LookupBranches results with branch-specific
errors and make singular service and store lookups specialize their
batch APIs.
Propagate MissingKeysError indexes with successful state responses
so branch classification needs no individual rereads.
Unexpected storage failures abort the batch.
Share metadata interpretation, upstream verification, and deleted-branch
handling with the existing lookup path.

Local branch filters select literal names and slash-delimited prefixes,
as used by branch lookup and sync deletion.
If filter arguments exceed the Linux, macOS, or Windows process limit,
retry without filters and match literal prefixes locally.
Retry only before any branch has been emitted,
preserving ordering and worktree metadata.

On the macOS arm64 demo repository with 103 local branches, hyperfine
uses 3 warmups and 15 runs of log short and log long, each with
--all --json.
Compared with the parent storage-support commit:

Listing	Parent mean +/- SD (s)	New mean +/- SD (s)
Short	1.601 +/- 0.299	1.550 +/- 0.353
Long	1.816 +/- 0.343	1.253 +/- 0.221

The short-listing difference is within the observed variation.

[abhinav]

This change is part of the following stack:

• git: Add streaming batch object reads #1480
  • storage: Add batch reads to the backend contract #1481
    • spice: Batch branch state and head loading #1489 ◀
      • log: Reuse loaded branch state in listings #1490

_{Change managed by git-spice.}