Adaptative concurrency improvements - fair sharing expectation

[coyotte508]

While implementing adaptative concurrency on huggingface.js (huggingface/huggingface.js#2350), fable 5 pointed out potential avenues for improvement in xet-core:

Ours is much simpler (throughput-delta AIMD), which fits a JS lib; but two findings may be useful upstream for xet-core:

1. No byte/memory budget: xet-core's controller is purely permit-count based. With v2 entries up to a whole xorb (~64MB compressed) and up to 64 permits, worst-case buffered memory is unbounded by anything except the permit count. Deriving a byte budget from the reconstruction (entry sizes are known upfront) would give a hard memory envelope — particularly relevant for hf_xet on small machines.
2. Zero-error saturation: xet-core's model treats linear RTT growth with concurrency as expected fair sharing (x_eff = size × N), so on a capped link it plateaus via the 60s target-RTT ceiling rather than detecting that aggregate goodput stopped improving. Our US-VPS data shows fixed high concurrency actively hurts on such links while all requests succeed. A cheap aggregate-goodput probe ("did the last +1 increase total throughput?") could let it settle lower, reducing both client contention and server connection load.

Especially 2. - fair sharing expectation