feat(speculation): allocator contract and sticky impl

[behinddwalls]

Summary

Why?

Ranked candidates need a budget policy: something must decide which paths get the queue's limited concurrent-build slots, without double-funding paths that are already running or finished.

What?

Adds submitqueue/extension/speculation/allocator — the budget-spending contract the standard Speculator hands its candidate stream to — plus the sticky implementation and mocks.

The Allocator owns terminal-path suppression: it reconciles the stream against the stored path sets by path ID, so an in-flight candidate keeps the slot it already holds and a terminal one is skipped. Budget accounting reads path status and nothing else — pending, building, and cancelling paths hold a slot until their builds actually stop — and the unit is deliberately coarse: one slot per concurrent build.

sticky fills only free slots and never preempts a running build; its policy counterpart is a preempting allocator. Output is all-or-nothing (a partial list would fund an arbitrary prefix of the ranking), and a stored path with no status fails fast rather than guessing whether it holds a slot.

Test Plan

✅ bazel test //submitqueue/extension/speculation/allocator/... — terminal candidates skipped per status, in-flight candidates kept, budget exhaustion, context cancellation, corrupt-status failure.

✅ make fmt, make gazelle, make mocks

Issues

[sbalabanov]

critical comment about failing fast

[behinddwalls]

critical comment about failing fast

i think we need log but not fail as there is no way to get rid of it afterwords; i will take care of it separately

ooo