v0.16.1

Patch release. Replaces bare integer arithmetic in Position with
saturating variants, matching the pattern already used in
Portfolio::execute_fill. Found by fuzz_execute_fill (added in v0.16.0
as part of the property-based testing pass after the v0.16.0 ship).

Fixed

• Position::market_value overflow (src/portfolio/position.rs:95):
  self.quantity * price previously panicked on i64 overflow.
  Replaced with saturating_mul. Reachable via the public
  Portfolio::rebalance_simple API; crash artifact preserved at
  fuzz/artifacts/fuzz_execute_fill/crash-330fb9097052e84d78571d06b840334680573582.
• Position::unrealized_pnl overflow (position.rs:104): same root
  cause; both the subtraction and multiplication now saturate.
• Position::apply_fill overflow (lines 55, 58, 69, 80, 87): all
  intermediate products/sums switched to saturating variants. Real-world
  trigger requires extreme quantity × price products; bug was latent
  pre-v0.16.0 but only surfaced after the fuzz harness was added.
• Portfolio::total_equity_from_price_map overflow (src/portfolio/mod.rs:430-435):
  bare .sum() over pos.market_value(price) and bare self.cash + position_value
  could panic when many large positions accumulated. Replaced with saturating_*.
  Found by fuzz_execute_fill re-run after the position.rs fix landed (crash artifact
  crash-6bafbd4544cfff56e4ac3e2d68e18ebeecf6c2d7).
• Portfolio::rebalance_simple arithmetic (src/portfolio/mod.rs:238, 285, 346, 385):
  defensive saturating_* replacements at five other i64 arithmetic sites in
  the rebalance / equity / price-mid paths. Each was a latent overflow risk
  reachable through the public API under extreme inputs.
• Portfolio::snapshot total_realized_pnl overflow (src/portfolio/mod.rs:397)
  [Inference]: bare .sum() over pos.realized_pnl fields — same pattern as the
  total_equity_from_price_map bug; replaced with saturating_add fold. Not in the
  original fuzz crash path but reachable from the same extreme-input scenarios.

Added

• tests/regression_position_overflow.rs: integration tests that
  assert each previously-crashing input sequence now returns a saturated
  i64 instead of panicking. Replays the libfuzzer crash artifact by hand.