feat(orchestrator): compose per-queue speculators and turn speculation on - #508

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## Summary

### Why?

Everything below this commit landed inert: the orchestrator passed the speculate run a placeholder that proposes nothing, so no path was ever funded and no speculative build started. The machinery is all reviewed; nothing configures it per queue or switches it on.

### What?

This is the activation switch. Per-queue profiles gain a `Scorer` and a `Speculator`: each queue's speculator is composed from its own scorer as `standard.New(bestfirst.New(scorer), sticky.New(budget))`, with a factory adapter beside the existing ones so routing stays in the wiring layer, and `main.go` swaps the placeholder for `profiles.SpeculatorFactory()`.

The scorer profiles are the policy knobs: the baseline scores everything 0.5, test-queue buckets by lines changed, and e2e-test-queue exercises the composite scorer. Every scorer is wrapped by `scorerfake` so a change URI carrying a failure marker forces a scoring error end-to-end. The build budget is a wiring-level constant (4 concurrent builds per queue) with a TODO to move it onto `entity.QueueConfig`.

With this commit, paths are funded, builds run per path, and batches finalize from their paths — the whole stack goes live in one revertable step.

## Test Plan

✅ `bazel build //...`, `make fmt`, `make gazelle`

✅ `make e2e-test` — the full pipeline lands changes end-to-end with per-path speculation on.