Split generic Backoff from ExponentialBackoff

[hwesterb]

Summary

• loosen Backoff to the cross-context delay-policy intersection
• add ExponentialBackoff for capped geometric growth with configurable jitter
• let transient Retry select the concrete child while StateLock composes with the generic family
• remove the false technical Backoff dependency from negotiation Yield
• regenerate the design manual, vocabulary root, hash cascade, and paper using the existing typography

The vocabulary grows from 452 to 453 patterns. The root changes from b7c42bc564f5a8d2ac3cb6140430e9d98feb82a8f9b943f550f554e9ba6360b5 to 901130d88dab244cc0d4afc149c5e6eeb9c9565e117c468a8e5326287be8fefa.

Verification

• 382 tests passed
• all 453 exported hashes valid
• clean 453-pattern rebuild produced zero JSON drift
• Ruff lint and format checks passed
• paper compiled to 37 pages with 0 undefined references
• no website, static asset, stylesheet, or typography changes

Closes #64