fix: use CLOB_HOST constant in examples (#27)

[MackDing]

Fixes #27.

The examples hardcoded https://clob-v2.polymarket.com, but per the v2 migration docs (https://docs.polymarket.com/v2-migration), as of April 28 2026 the CLOB endpoint is https://clob.polymarket.com.

This PR:

• Adds a top-level pub const CLOB_HOST: &str = "https://clob.polymarket.com"; in src/lib.rs.
• Updates all examples/clob/** to import and use CLOB_HOST rather than the stale literal.

cargo check --examples --all-features passes locally.

---

Note

Low Risk
Only changes example default URLs and introduces a new public constant; no core client logic or request/response handling is modified.

Overview
Updates the CLOB Rust examples to stop hardcoding the legacy https://clob-v2.polymarket.com endpoint and instead default to a shared CLOB_HOST constant (while still allowing override via CLOB_API_URL).

Adds pub const CLOB_HOST: &str = "https://clob.polymarket.com"; to src/lib.rs and switches all examples/clob/** client construction to reference it for consistency with the v2 migration docs.

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