Add orphan-detection pre-filter gate to short-circuit v2_backfill_orphan kills

Create a new pre-filter gate `orphan_check` that runs BEFORE filter_score. It inspects each candidate hypothesis for (a) the v2_backfill_orphan tag, (b) missing required v2 fields (resolution_criterion, thesis_anchor, reachability_window), or (c) presence in the S148 orphan ledger. On match: short-circuit to KILL with reason `orphan_pregate` and skip filter_score entirely. Wire the gate into pipeline.js between candidate ingest and the filter_score move. No schema changes to the hypotheses table — only an additional gate stage.

The 7× v2_backfill_orphan_S148 kills currently consuming filter_score budget shift to a zero-cost pregate KILL; filter_score spend on orphan candidates drops to ~0; downstream kill_reason distribution loses the v2_backfill_orphan_S148 bucket within filter_score post-mortems.

Run for 7 days after enabling. If filter_score still records ≥1 v2_backfill_orphan_S148 kill, the gate is mispositioned or its detector misses the orphan signature. If orphan_pregate rate exceeds 7/week (the historical rate), the detector is over-firing on non-orphan candidates and the gate is misdesigned.

Rejected by filter_score. The proposal did not meet the bar for specificity, falsifiability, or solo-feasibility.