Gate role-changing fixed reversals by free margin

[luisleo526]

What changed

• Recheck omitted-quantity fixed market entries when an earlier sibling fill changes them from a same-direction order into a reversal.
• Compare the augmented close-plus-open transaction against free margin at the actual fill.
• Add boundary and scope tests for equality, explicit quantities, non-100% margins, percent-of-equity sizing, ordinary adds, and slippage.

Root cause

Two market entries can be queued for the same next tick. The second may be same-direction when created, then become a reversal after the first fills. The engine admitted that stale-role order without rechecking the larger reversal transaction, producing an extra flip.

Impact

The validated GB2 strategy moves from moderate to strong: engine trades 3153 → 2685, with all 2617 TV trades matched and count delta reduced 536 → 68. The full campaign sweep had one upward tier move and no downward moves.

Validation

• Focused margin-admission test: 72/72 assertions
• Full engine suite: 83/83
• Corpus validation: 252/252; tiers unchanged at 239 excellent / 12 strong / 1 anomaly
• Full 412-strategy sweep: 274 excellent / 79 strong / 32 moderate / 15 weak; 1 up / 0 down
• Independent semantic review: GO

Production diff SHA-256: c204dddb8a9126c8bed503eb2a425d1e7051a1056a9bc37f69bdbb5810330302