feat(clob): support PolyV2 position IDs

[kartojal]

Summary

• add explicit position_id support for limit and market orders with exclusive token/position validation
• route position-backed orders through Exchange V3 signing on Polygon and Amoy, bypassing server version and neg-risk lookups
• propagate position IDs through order books, tick sizes, fee metadata, amount calculation, signing, serialization, and submission
• support position IDs in market-data reads while preserving the protocol token_id and tokenId wire fields
• add integration tests, request serialization coverage, API documentation, README guidance, and a changelog entry

Reference

Rust implementation of the behavior introduced in Polymarket/clob-client-v2#104:
Polymarket/clob-client-v2#104

Validation

• cargo test --all-features
• cargo test
• cargo build --all-targets --all-features
• cargo +nightly-2025-11-24 fmt --all -- --check
• cargo +1.88.0 clippy --all-targets --all-features -- -D warnings
• cargo +1.88.0 clippy --all-targets -- -D warnings
• git diff --check

---

Note

Medium Risk
Touches order construction, EIP-712 signing, and contract selection—errors could produce unfillable or wrongly signed orders—but behavior is heavily tested and token-only paths are largely unchanged.

Overview
Adds Polymarket V2 position-backed trading alongside existing CTF token orders. Limit and market builders gain position_id(); callers must supply exactly one of token_id or position_id.

Position orders always use Exchange V3: EIP-712 domain version "3", exchange_v3 on Polygon/Amoy, same V2-shaped wire payload (tokenId carries the position ID). They skip GET /version and neg-risk during build/sign. Token orders keep V1/V2 auto-detection and version-mismatch retry on post.

Signing and types: OrderPayload::V3, new_v3, and v3() accessors; V3 shares serialization with V2. Poly1271 is documented for V2/V3 orders.

Market data and metadata: Read paths (midpoint, books, tick size, fees, market_by_token, etc.) treat position IDs like tokens, still serialized as token_id on the wire.

README, module docs, changelog, request docs, config tests, and integration tests cover position limit/market flows, mutual exclusion of IDs, and serialization.

^{Reviewed by Cursor Bugbot for commit 4d2cc8a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cesarenaldi]

The implementation matches the intended minimal clob-client-v2 port. The remaining test-fixture and terminology points are non-blocking.