counterfactual: pure fixture assembler — replayable cases from the raw-context corpus

[JSONbored]

Problem

Counterfactual sub-epic phase 2. The replay harness needs its input shaped: from a labeled corpus, select and shape exactly the cases that are replayable under the design contract — pure selection, no AI calls, no IO.

Requirements

⚠️ Required pattern. Engine-pure module in packages/loopover-engine/src/calibration/, mirroring the corpus builders' discipline (#8083 shape); root vitest mirror suite per the engine blind-spot rule. Implements the fixture/sampling contract types from the design issue verbatim — do not invent alternatives.

• assembleCounterfactualFixtures(cases, contract): filters to replayable (diff present + bounded, label present), applies the SEEDED deterministic sample when over budget, and emits fixtures carrying {targetKey, label, boundedInputs, provenance} — provenance distinguishes live-captured vs backfilled raw context (calibration: backfill phase 2 — GitHub-truth reversed labels (retro successor scan) + raw-context re-fetch #8170's tags) so results can be segmented by capture era.
• Skip accounting in the return shape (why each non-replayable case was excluded), mirroring how the Extend the backtest check to logic/regex changes via a dedicated CI job, using captured raw context #8139 logic backtest counts skipped pre-capture cases rather than silently dropping them.
• Deterministic: same corpus + contract ⇒ same fixture set.

Deliverables

• Assembler + types exported from the engine barrel, 100% line+branch mirror coverage (eligibility arms, seeded sampling determinism, skip accounting, era segmentation)

Links & Resources

The sub-epic (parent), the design issue (blocked-by), packages/loopover-engine/src/calibration/backtest-corpus.ts

Boundaries

Pure only. No fetches, no model calls, no store reads.

[linear-code]

JSO-5704