fix: harden market ID generation for high-volume environments

[dreamgene]

Closes #178

Summary

• harden market ID allocation with checked incrementing
• fail explicitly on ID overflow and on pre-existing ID collisions
• keep the existing sequential storage model and storage layout intact

Testing

• cargo fmt --all
• cargo test -p predict-iq (currently blocked by unrelated pre-existing compile failures outside the Potential Market ID Overflow in DataKey::Market #178 scope)

Notes

• added targeted tests for sequential uniqueness, overflow, collision, and a 1,000,000-ID allocator simulation
• the repository currently has unrelated compile issues in other modules (for example duplicate contract exports and mismatched module/type usage), so full CLI checks are not green on this branch yet

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