fix(git-cli-proxy): bound concurrent page serves so blob-heavy windows cannot stack past the memory limit

[aleksdotbar]

Cherry-pick of #3032 into release-2026.08.

Why. Nothing bounds how many page serves run at once: heavyOpsConcurrency caps clones/fetches/repacks, but each serve spawns git children whose memory scales with the window's blob bytes, so enough concurrent blob-heavy windows OOM-kill the pod — and now that long preparations are held instead of bounced, heavy serves actually run long enough to stack.

What changed. A serve_concurrency semaphore (config-required like its heavy sibling; chart value cache.serveConcurrency, default 4) gates the serve phase of every data endpoint at the shared read_snapshot choke point. The permit is taken after open() so a slot is never spent waiting on a preparation, and excess requests wait in-connection; a wait that outlives the preparation ceiling answers a typed 429 with its own quota subject and rejection metric.

Verified. Clean cherry-pick. cargo test -p git-cli-proxy on this branch (215 lib + 20 boot), helm contract suite (27).

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