Generic in-process scheduler for the expensive PDF-extraction child process.
A pool runs at most `slots` jobs, keeps at most `queue` waiting, and refuses a
waiter past `wait`. Do() applies six rules in order: join an existing running
or queued job (no slot, allowed while the caller holds another, fn may be nil);
refuse a caller that already has a job in flight; take a free slot, else queue
(first-come-first-served, else ErrBusy); run fn under context.WithoutCancel so
the flight outlives a caller that leaves; broadcast the result to every sharer
with no caching and a recovered panic returned as an error. Running counts only
jobs holding a slot; Queued counts the line; Waiting(key) counts joiners.
Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>
kyleandornith-1.5-35b-a3b
bb9a6544d1