Stage 10a — liquidation margin math (pure compute)

First sub-stage of the liquidation engine. Pure compute: no I/O, no
state, no async. Same shape as `openhl-funding` Stage 8b — a
deterministic function bank over snapshots, with i64 + saturating
arithmetic so validators reach the same MarginHealth on the same
inputs.

What's in:

- `LiquidationParams { initial_margin_bps, maintenance_margin_bps,
  liquidation_fee_bps }` + `hyperliquid_default()` (10% / 2% / 1.5%)
- `AccountSnapshot { account, position_size, avg_entry, collateral }`
- `MarginRatio` newtype scaled by `MARGIN_SCALE = 10_000` (bps)
- `MarginHealth` enum: `Safe`, `AtRisk`, `Liquidatable`, `Underwater`
- `CloseOrderSpec { account, side, qty }` — bridge encodes this as
  `clob::Action::SubmitMarket`
- Six pure compute functions:
    notional_value, unrealized_pnl, account_equity,
    margin_ratio, margin_health, close_order_spec
- 21 unit tests + 3 proptests (long monotonicity in the levered
  regime, short monotonicity, ratio determinism)

What's out (deferred to next sub-stages):

- Stage 10b — insurance fund state machine, deficit absorption,
  fee credit
- Stage 10c — multi-account scanner, CLOB integration, position
  registry hook
- Auto-deleveraging (Stage 10d, optional)

Two design notes worth pinning:

1. `AccountSnapshot` carries `avg_entry` and `collateral` that the
   funding `Position` doesn't — liquidation needs unrealized PnL,
   funding doesn't. Bridge layer assembles the snapshot per account
   per tick. The two crates stay independent.
2. Long ratio monotonicity holds only when `entry × size > collateral`
   (the levered regime). When collateral dominates notional, the
   `collateral / notional` term inverts the relationship — but that
   case is uninteresting for liquidation (the account can never be
   liquidated), so the proptest excludes it via `prop_assume!`.

Author: psyto

Cargo.lock

@@ -8,6 +8,11 @@ repository   = { workspace = true }
 authors      = { workspace = true }
 
 [dependencies]
+openhl-clob    = { path = "../clob" }
+openhl-funding = { path = "../funding" }
+
+[dev-dependencies]
+proptest = { workspace = true }
 
 [lints]
 workspace = true