fix(clob): enforce backend precision limits for market orders

[Haut]

Fixes #114

The existing implementation in OrderBuilder<Market, K>::build() calculated maker/taker amounts using:

(raw_amount / price).trunc_with_scale(decimals + LOT_SIZE_SCALE)

Where decimals is the tick size scale (1-4) and LOT_SIZE_SCALE is 2. This produces amounts with 3-6 decimal places depending on tick size, violating the backend's strict precision requirements:

Order Side	Maker Asset	Max Precision	Taker Asset	Max Precision
BUY	USDC	2 decimals	Shares	4 decimals
SELL	Shares	2 decimals	USDC	4 decimals

Changes Made

1. Added RoundingStrategy enum with three variants:

   • Down (default) - Truncate towards zero (safest to avoid overspending)
   • HalfUp - Standard rounding (round 0.5 away from zero)
   • Up - Round away from zero

2. Added .rounding_strategy() builder method on OrderBuilder<Market, K> allowing users to configure rounding behavior per-order

3. Implemented context-aware rounding that respects the backend's precision matrix:

   • BUY orders: Maker (USDC) → 2 decimals, Taker (shares) → 4 decimals
   • SELL orders: Maker (shares) → 2 decimals, Taker (USDC) → 4 decimals

4. Added zero-amount validation - Returns Error::validation() when rounding produces zero amounts (e.g., trying to sell 0.001 shares when limit is 0.01)

5. Exported RoundingStrategy from the clob module for user access

Tests

• Integration tests for market order building pass
• Integration test against live/testnet API

[Haut]

@chaz-polymarket this fixes #114 for me. adds an optional builder method for setting rounding and matches the other python / ts clients from what I can see. tested live and seems to work for everything I tried. not sure if you want to get in before 0.4 but its here if needed

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.96%. Comparing base (0169712) to head (1214f60).

Files with missing lines	Patch %	Lines
src/clob/order_builder.rs	94.93%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
+ Coverage   85.81%   85.96%   +0.14%     
==========================================
  Files          32       32              
  Lines        4885     4965      +80     
==========================================
+ Hits         4192     4268      +76     
- Misses        693      697       +4     

Flag	Coverage Δ
rust	85.96% <95.00%> (+0.14%)	⬆️

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[2msd]

Is it possible that this affects limit orders too?

If I try to sell an amount of shares with a limit oder, I get "not available funds". If I subtract 0.01 from the amount, the sell order goes though.

If this is another issue, I will gladly open a new ticket.