fix(global-db): prove collision fast-path work

[ScriptedAlchemy]

Summary

Replaces #653 from its last clean reviewed ancestry. The old PR branch was concurrently advanced with a rejected durable-counter/schema implementation, so this corrective PR carries only the behaviorally verified collision-authority patch.

• records terminal identity-collision refusal and cursor coverage atomically
• keeps only real-backed different-output provenance collisions convergent
• preserves hard failure for ghost/same-output corruption
• proves terminal re-admission never accesses the retained observation row and dispatches exactly one observation runtime command
• preserves the corrupt-row tripwire and hides the observations table so even an ignored bare-column read fails
• removes the former test-only identity-digest probe and feature wiring

Incoming candidate identity and payload digests are canonically produced upstream by DurableObservationV1::new; the marker branch consumes their private-field validated values and performs no digest operation of its own. This PR does not add redundant hashing for instrumentation. Runtime measurement is scoped to the observation adapter's actual read/submit dispatch boundaries, with a two-command full-path positive control and one-command terminal-path assertion.

Verification

Exact base: 26087ea7ae69baa2c52d5ae694394e82024c0e09
Exact head: 8a2e22ba62243757e4d98ba49abe33462474b6de

• RED: renamed exact regression failed 0/1 because the mounted positive control observed 0 rather than 2 observation commands; SHA-256 a20b10fc7e4e845a8e1e1c27e3d4099d7f76cc687b71a9c72d45a6e8b7906853
• GREEN: exact regression 1/1, 402 filtered; SHA-256 0e8d62d26a203c8c4d722cfa475bcfd1f8f699ea0fe021a9c99f8eff6c43e5b7
• collision module: 18/18; SHA-256 49d930bdc8ae4d07236d4e582cad3a38d3a7f10619feca0d1c1b5daedcfcc17c
• scoped global-db Clippy, locked, -D warnings: green; SHA-256 fdc9756e32763a471a76a64c2cc1c5f47e1582212a6043c77ec0d2f18d129b37
• focused format, diff-check, and commitlint: green

No force push and no merge into master.

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