feat(orchestrator): re-plan the queue from the Speculator each run - #506

## Summary

### Why?

The speculate controller had no speculation in it: it advanced one batch at a time along a single hard-coded chain — every dependency assumed to pass — and nothing ever called the Speculator, the Generator, or the Allocator that had been built for it. This wires that machinery into the pipeline: a queue's paths are chosen by a swappable extension within a build budget, and each is built against only the dependencies it assumes will succeed.

### What?

Every message is a dirty signal naming a batch; the controller re-plans that batch's whole queue from a single read: read the state, cancel paths whose assumptions a finished dependency has proven wrong, ask the Speculator, filter its proposals, dispatch what survives. Nothing carries over between runs, so duplicated or reordered signals are harmless and a later run repairs whatever an earlier one left half-done. `doc.go` explains the model in plain terms — no vocabulary section, every term is defined where it is used.

The path set keeps exactly one writer — this run. The build stages record what CI did on per-build records; the run folds those into the set and alone decides each path's status. Pending paths are re-dispatched every run until their build is seen running, dispatches partition by batch so heads proceed in parallel, and cancelling paths need no dispatch at all: the poll loop reads the stop off the set and enacts it.

Speculation lands inert. The wiring layer passes a placeholder Speculator that proposes nothing, so the run executes end to end but funds no paths; composing real per-queue speculators and turning the feature on is the wiring change at the top of this stack. Batch outcomes still come from the legacy per-batch finalizer, which waits on every dependency — strictly stricter than path-aware finalization, so the system stays correct until the next commit replaces it.

## Test Plan

✅ `bazel test //submitqueue/orchestrator/controller/speculate/...` — assumption checks and proposal filtering are table-driven; run tests cover funding a first path, re-dispatching pending paths, broken-path cancellation, build results recorded onto paths, lost CAS races skipped rather than failed, and Speculator errors abandoning the run.

✅ `make fmt`, `make gazelle`

# Conflicts:
#	service/submitqueue/orchestrator/server/BUILD.bazel
#	service/submitqueue/orchestrator/server/main.go
#	submitqueue/orchestrator/controller/speculate/BUILD.bazel
#	submitqueue/orchestrator/controller/speculate/speculate.go
#	submitqueue/orchestrator/controller/speculate/speculate_test.go

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#	modified:   submitqueue/orchestrator/controller/speculate/speculate.go
#	modified:   submitqueue/orchestrator/controller/speculate/speculate_test.go
#	modified:   submitqueue/orchestrator/pipeline.go
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# Conflicts:
#	service/submitqueue/orchestrator/server/main.go

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# Changes to be committed:
#	modified:   service/submitqueue/orchestrator/server/BUILD.bazel
#	modified:   service/submitqueue/orchestrator/server/main.go
#	modified:   submitqueue/orchestrator/BUILD.bazel
#	modified:   submitqueue/orchestrator/controller/speculate/BUILD.bazel
#	new file:   submitqueue/orchestrator/controller/speculate/check.go
#	new file:   submitqueue/orchestrator/controller/speculate/check_test.go
#	new file:   submitqueue/orchestrator/controller/speculate/dispatch.go
#	new file:   submitqueue/orchestrator/controller/speculate/doc.go
#	new file:   submitqueue/orchestrator/controller/speculate/run.go
#	new file:   submitqueue/orchestrator/controller/speculate/run_test.go
#	new file:   submitqueue/orchestrator/controller/speculate/snapshot.go
#	new file:   submitqueue/orchestrator/controller/speculate/snapshot_test.go
#	modified:   submitqueue/orchestrator/controller/speculate/speculate.go
#	modified:   submitqueue/orchestrator/controller/speculate/speculate_test.go
#	modified:   submitqueue/orchestrator/pipeline.go
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