perpetuals: fix mark-price scaling bug, add ReduceCollateral, expand test coverage

[Copilot]

All four ledger tests were failing because test mark prices were multiplied by PRICE_SCALE — the internal fixed-point factor for quantities, not prices. This caused quantity_from_collateral to always round to zero, returning InvalidCollateral on every open_position call. The liquidation test had a second bug: a 4% price drop on a 5× leveraged position leaves equity well above the maintenance threshold, so liquidation never triggered.

Bug fixes

• Mark price scaling: Remove * PRICE_SCALE from all test mark prices. Mark prices are plain quote-asset integers (50_000 for $50k BTC); PRICE_SCALE = 1_000_000_000 is only the precision divisor in quantity arithmetic.
• Liquidation threshold: Drop test price from 48_000 to 40_000. At 5× leverage + 5% maintenance margin, equity at 48k is 800 vs. maintenance 240 — not liquidatable. At 40k equity reaches 0.

New: ReduceCollateral operation

Symmetric counterpart to AddCollateral (wire tag 6). Owners can withdraw excess margin, with a guard that simulates the post-reduction position and rejects if it would fall into liquidatable territory:

PerpetualOperation::ReduceCollateral { position, amount }
// Returns Err(CollateralReductionWouldCauseLiquidation) if the reduced
// position's equity would be ≤ maintenance requirement at current mark price.

Added CollateralReductionWouldCauseLiquidation as a distinct error variant (separate from CollateralUnderflow which covers arithmetic underflow when amount > collateral).

Test coverage (6 → 25)

• Ledger: unauthorized add/close, nonce mismatch, short-side profit, add+reduce collateral round-trip, blocked reduction near liquidation threshold, underwater-position close prevention, open-interest accounting across close + liquidation
• Transaction codec: roundtrip for all 8 operations, encode_size matches wire length, stable discriminant constants, signed-transaction roundtrip + verify
• Genesis: JSON roundtrip, market creation from genesis, zero-price and invalid-hex rejection

[faddat]

@copilot fix the actions that don't pass

[Copilot]

@copilot fix the actions that don't pass

Fixed. The fmt job was failing due to formatting violations in genesis.rs, ledger.rs, and transaction.rs — a trailing blank line in two files and a line-length issue in the other two. Running cargo fmt resolved all of them (commit perpetuals: fix rustfmt formatting violations).

[faddat]

@copilot fix actions that don't pass

[distractedm1nd]

Closing in favor of #79