Value update floor: 0.3, with prod observability

[nambok]

Summary

The prod bisect for the reported supersession bug: with 0.32.1 live, the exact identical frame pair still coexisted through the explicit store path. The engine logic is proven by tests; the remaining gate is the cosine floor. The config's own measurements put real value pairs at 0.51 to 0.80, and Cohere 1024 evidently lands this pair below 0.5, inside the old floor. The floor moves to 0.3, still above genuinely off topic collisions, and a frame match rejected by the floor now logs its similarity so production data keeps the default honest.

Changes

• value_update_min_similarity default 0.5 to 0.3, doc updated with the prod observation
• Frame matches below the floor log an info line with the measured similarity instead of vanishing silently
• Tests: an identical frame pair at cosine 0.4 must fire; the below floor negative moves to 0.2 and asserts the current action shape (it had gone vacuous asserting the retired CreateEdge form)

Verification

• 104 cognitive tests green, full workspace zero failures, clippy clean
• Post release: platform pin bump, deploy, and the reported repro re-run against prod; the new log line proves or refutes the floor diagnosis either way