feat(speculation): add best-first path generator

[sbalabanov]

Summary

Intent:

• Give the default speculator a lazy global stream of candidate paths ranked by the likelihood that dependency assumptions hold.
• Avoid eager exponential path generation while preserving resolved dependency facts and deterministic ordering.

Changes:

• Add generator and iterator contracts plus a best-first implementation using log-space extend/swap enumeration and a global heap merge.
• Add generated mocks, Bazel targets, comprehensive tests, and a focused design RFC with a worked example.

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Test Plan

Issues

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