Subscriptions: share authorized evaluations with bounded coalescing #175

Problem

Identical subscriptions recompute independently, multiplying work. Sharing must respect rule generation and visibility rather than merging different authorization scopes.

Evidence

Source: 5c1f313379a5cb2995a43a66ddba0b5cc9798b7f (inspected 2026-10-03). These are source findings; the acceptance scenarios below still need implementation and execution.

• src/protocol/subscription/reevaluate.rs:76: Each refresh executes the query again.
• src/protocol/subscription/reevaluate.rs:88: Rows are keyed by canonical JSON before diffing.

Acceptance criteria

• Key shared evaluation by canonical query plan, KG, rule generation and equivalent visibility scope; fence revocation at fanout.
• Use collision-safe tuple identity with equality checks, preserve dependencies/pending changes, and release shared state after the final subscriber.
• Bound coalescing by measured latency headroom; no unmeasured default debounce delay.
• Test distinct auth scopes, rule changes, ID reuse, unrelated writes and hash collisions. Measure 1/64/100 identical and distinct subscribers with Performance: baseline and mandatory pre-PR regression pipeline #163.

Delivery

Plan item: E01. R3 - Streaming SDK subscriptions.

Dependencies / related work: #137, #174, #163.

Contributes to #133; completing this slice alone does not close the broader issue.