PERCENTAGE_SPLIT: implement engine's 1-in-9999 recursion edge case in SQL

The pure-SQL `PERCENTAGE_SPLIT` hash diverges from the engine on the ~1/9999 inputs where the bare hash mod 9999 == 9998 (the engine recurses with doubled input; this implementation skips).

Real-world impact at typical thresholds: ~0.005% false-negative rate on the count of identities matching a percentage-split segment. At 870M with threshold 50, that's ~22k false-negatives across the env — a rounding error on a count-badge UI but a measurable bias if anyone uses the count for billing or contract decisions.

## What to ship

Implement the recursion as a `CASE WHEN bare_hash_mod = 9998 THEN ELSE END` wrapper in the inline SQL. The recursive hash uses doubled input: `seg_key || ',' || value || ',' || seg_key || ',' || value`. Cap at 2-3 iterations (engine recurses arbitrarily but in practice the second iteration almost always lands at non-9998).

## Why deferred

Sub-0.005% bias on a UI count is below the threshold that customers care about. Defer until a customer reports a discrepancy or until the engine's bucketing semantics change.