exec_unified: add placer.py (SmartPlacer pre_submit seam) + mutation-litmus tests
- MarketSnapshot: best_bid, best_ask, spread_bps, tick, step (Decimal, frozen, validated)
- PlacementPlan: limit_price (quantized to tick), post_only=True
- pre_submit: returns None for TAKER, spread gate failure, or quantize-cross; otherwise PlacementPlan
- Spread gate uses live spread_bps (fixes dead _spread_allows_maker)
- Quantize to tick BEFORE returning (ROUND_HALF_EVEN per §4-15)
- Cross-after-quantize guard rejects if tick quantization crosses book
- 33 mutation-litmus tests: spread-gate flip, TAKER gate, cross-after-quantize all go RED
- Router.PROTECT wants_placement=True (at touch)
- Pure stdlib+Decimal, zero I/O/venue