docs(research): recover lineage depth-64 provenance

[Skywind5487]

Summary

Completes the focused archaeology requested by #51 without changing production lineage behavior.

Adds research/session-lineage/depth-fuse-provenance.md, pinned to dev SHA 311bf7d6d28b204f0aa977ddcd05d44141d2d4ba.

Key result:

• 2732c47e28fbf7aaea97bd8c5cf82045a4c34159 introduced the surviving lineage_depth_cap=64 together with the fork-local recursive SQL winner path.
• The Python parent resolver that commit replaced used a visited set and had no numeric depth cap, so exact-64 behavior was not inherited from the immediate production predecessor.
• Cycle protection is independently implemented (SQL path-membership guard; import hardening uses seen/ancestors).
• No primary source recovered a rationale for the literal number 64 or a product rule that only N ancestors count.
• Current acyclic cap hit silently resolves to the deepest reached node, which is seed-distance-dependent. The artifact separates that observed fallback from the safety intent future Session search lineage reuse: choose the simplest query-local traversal strategy #45 contenders must preserve.

The research therefore narrows the minimal contract to cycle termination, deterministic malformed/missing-edge handling, and bounded pathological acyclic work, without selecting a replacement mechanism.

No production code or tests are changed.

Closes #51