feat(orchestrator)!: finalize batches from their speculation paths - #507

## Summary

### Why?

Speculation was building the right things and merging on the wrong rule. A head still waited for *every* dependency to succeed before it could merge — the same rule as before any of this — so a batch built without a slow neighbour sat behind that neighbour anyway. The paths were being earned and then ignored. This is the commit that collects them.

### What?

A head merges once one of its **passed** paths has had every dependency it *assumed would succeed* actually merge; assumed-failing and ignored dependencies impose no wait. That is the whole speedup: the head waits on what its passing build was actually stacked on, not on its full dependency list. A head fails only when no funded path has a future left, and the winner's siblings are superseded — cancelled to free their slots.

Outcomes are reached in `finalize`, before the Speculator is asked, and committed one generation at a time, so a cascade (A fails → B's last path breaks → B fails → maybe C too) resolves in a single run without ever enacting a dependent of an outcome whose own write lost its compare-and-swap. A batch decided by a cascade gets a recovery signal before it turns terminal, since no retry of the triggering message would ever revisit it.

Cancellation joins the run: a cancelling batch is just another batch the run walks — its live paths are marked cancelling, the poll loop stops their builds, and whichever later run sees them all stopped drives the batch to Cancelled. A cancelling path with no build link is cancelled immediately; there is deliberately no reservation state and no staleness bound, so a crashed dispatch can never keep a batch out of its terminal state.

The legacy per-batch finalizer is deleted; `speculate.go` keeps only admission and message routing, and the package doc gains the batch lifecycle, a worked example, and the finalize step.

## Test Plan

✅ `bazel test //submitqueue/orchestrator/...` — the merge rule is table-driven per assumption; failure, cascade commit ordering, and CAS-loss isolation are covered, and cancellation end to end: never-dispatched, live-build, no-paths, and lost-race cases.

✅ `make fmt`, `make gazelle`

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

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# interactive rebase in progress; onto ec05b712
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# Changes to be committed:
#	modified:   submitqueue/orchestrator/controller/cancel/cancel.go
#	modified:   submitqueue/orchestrator/controller/speculate/BUILD.bazel
#	modified:   submitqueue/orchestrator/controller/speculate/check.go
#	modified:   submitqueue/orchestrator/controller/speculate/dispatch.go
#	modified:   submitqueue/orchestrator/controller/speculate/doc.go
#	new file:   submitqueue/orchestrator/controller/speculate/finalize.go
#	new file:   submitqueue/orchestrator/controller/speculate/outcome.go
#	new file:   submitqueue/orchestrator/controller/speculate/outcome_test.go
#	modified:   submitqueue/orchestrator/controller/speculate/run.go
#	modified:   submitqueue/orchestrator/controller/speculate/run_test.go
#	modified:   submitqueue/orchestrator/controller/speculate/snapshot.go
#	modified:   submitqueue/orchestrator/controller/speculate/speculate.go
#	modified:   submitqueue/orchestrator/controller/speculate/speculate_test.go
#

# Conflicts:
#	submitqueue/orchestrator/controller/speculate/speculate.go

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# Last command done (1 command done):
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# You are currently rebasing branch 'preetam/speculation-finalization' on '9a490fe4'.
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# Changes to be committed:
#	modified:   submitqueue/orchestrator/controller/speculate/BUILD.bazel
#	modified:   submitqueue/orchestrator/controller/speculate/check.go
#	modified:   submitqueue/orchestrator/controller/speculate/dispatch.go
#	modified:   submitqueue/orchestrator/controller/speculate/doc.go
#	new file:   submitqueue/orchestrator/controller/speculate/finalize.go
#	new file:   submitqueue/orchestrator/controller/speculate/outcome.go
#	new file:   submitqueue/orchestrator/controller/speculate/outcome_test.go
#	modified:   submitqueue/orchestrator/controller/speculate/run.go
#	modified:   submitqueue/orchestrator/controller/speculate/run_test.go
#	modified:   submitqueue/orchestrator/controller/speculate/snapshot.go
#	modified:   submitqueue/orchestrator/controller/speculate/speculate.go
#	modified:   submitqueue/orchestrator/controller/speculate/speculate_test.go
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