(OPT) Replay identity-certified shared PLAN prefixes without repeated comparison payloads #899

The retained PLAN heapsort repeatedly compares observations inside the same immutable segment shared by sibling histories. Existing #875 shares physical backing; this follow-up would exploit that exact identity during comparison. It does not replace lexical order with hash order or change the sort schedule.

Discovery source: b8f2295. src/cleanup_plan/replay/ordinary_plan_history/producer.rs:246–318 creates shared arena segments. Existing Cursor::prepare in ordinary_plan_history.rs:245–285 validates links, ordering, depth and backing. comparison_owner.rs:318–501 and comparison_owner/paired_value.rs:36–104 contain the left-before-right preparation and work/cursor sequence.

After both existing preparations, recognize aligned identical segment indices within the same immutable arena. Execute a funded scalar replay state for that segment, preserving EVERY original observation/left-step/left-segment/right-step/right-segment work call individually and in order. Physically avoid observation references, slice-head/rest payloads and content comparisons for certified equal observations. Divergent segments and tails use the complete ordinary path. Final PLAN/HIR validation, heapsort states, terminal ordering and swaps stay independent and unchanged.

Acceptance:

• Freeze and independently compile the complete immediate comparator/cursor/macros/invoices; preserve every older oracle and source control. Register actual new children in all owning source unions.
• Invoice the complete new owner, identity detector, retained range/offset/length state, receivers, publication, control, fallback and drop costs before formation. Remove only physically absent payloads; keep all surviving conservative legacy invoices explicit. No fee/cap relaxation, baseline inflation, credit, work batching or crosscheck discount.
• Preserve exact malformed-link/depth discovery, left/right failure order, sticky byte/work refusal, Oracle/Selected authority and same-invocation borrowing. Do not prescan all paths or let a certificate survive mutation/swap invalidating its referent.
• Compare complete six-field diagnostics, work remaining/spent, swaps and full Drop on bounded every-work-cut fixtures. Cover long shared prefixes, distinct segments with equal contents, empty/deep segments, tails, first/last divergence, duplicates/strict prefixes, terminal differences, invalid links and both HIR representations.
• Exercise every new acquisition at exact/short/zero/floor, post-swap and sticky failure. Census actual identity hits/certified observations/fallback observations/replayed work and full new bytes; distinguish eliminated physical cursor operations from executed ones.
• Run unchanged whole 32-row caller and actual external codec/native/catalog owners at 67,108,864 bytes, plus only relevant focused checks and production Clippy. Never full suites/gates. A later refusal is progress, not admission.

The failed 6a839 unit prefix observed 598 prepared pairs and 51,870 rounds, but neither is a count of shared-segment observations. The existing heapsort is already O(P log P); this targets repeated long prefixes, not alleged quadratic pair enumeration. Opportunity is removed actual comparison/cursor payloads on identity-certified observations minus ALL new costs. No numeric saving, application-fit or token advantage is established. Astra performed read-only discovery; no tests/builds ran for this proposal.