docs(rfc): record the speculation path generation design

[behinddwalls]

Summary

Why?

The default Generator's design lives only in the bestfirst package README, written for readers of the implementation. The design deserves a decision doc: what the generator promises, how it ranks and lazily generates paths, and which alternatives were considered and rejected — including the forward decision-tree rewrite and the DAG-chain variant, so the reasoning is on record before anyone proposes them again.

What?

Adds doc/rfc/submitqueue/speculation-generator.md: a plain-English RFC for the existing best-first generator. It grounds everything in one worked queue — A, B, C fully connected plus an independent D — draws every batch's possible paths as literal trees with scores computed by plain multiplication, walks generation step by step (start from each batch's most likely path, flip outward, two follow-up variants per returned path), separates the multiplied score from the stored logarithmic ranking value, shows the same queue across two runs (a known outcome stops being a guess while path identity survives), and defines the edge cases and the deterministic tie order. Alternatives considered and rejected: enumerate-and-sort (2^n cost), a forward decision tree per batch (same output, more queue work, pure churn), and trees over DAG-derived dependency chains (batches store direct conflicts only, so per-run chain derivation destabilizes path identity — the dependency-closure problem). speculation.md links to the RFC from the Generator bullet and the extension-API pointers.