feat(speculation): standard composed speculator

[behinddwalls]

Add submitqueue/extension/speculation/speculator/standard, the standard Speculator. It funds the queue's most promising speculation paths first until the build budget is spent: candidates are considered in descending likelihood, in-flight paths stay funded rather than restarting, finished paths are never re-proposed, and everything below the budget cut waits for a later run. Where the ranking and the budget policy come from is swappable — the Generator ranks, the Allocator spends — so pairing bestfirst with sticky yields the default speculation policy, and changing either behavior means swapping a part, not writing a new Speculator.

standard adds no cancellation handling of its own and inherits it from the two parts; a test pins that a cancelled run yields the context error and no actions.

standard's behavior is documented next to the code in standard/README.md; the speculator README keeps only the extension contract.

Stack

1. refactor(entity)!: speculation paths carry assumptions, not bets #473
2. feat(speculation): generator contract and bestfirst impl #446
3. feat(speculation): allocator contract and sticky impl #450
4. @ feat(speculation): standard composed speculator #451