Feature/docs and tests batch

[favourawaku]

Summary

This batch addresses four GitHub issues: documentation for intent ID derivation (#82), two test improvements for contract behavior (#79, #81),
and CI infrastructure for mutation testing (#80).

Changes

#82 — Document the intent_id derivation scheme

• Added Intent ID Derivation section to README with exact preimage field ordering
• Documented hash function (SHA-256) and field types for off-chain solver tooling
• Included pseudocode example for reference implementations
• Cross-linked from lib.rs compute_intent_id implementation

#79 — Add test asserting expire_intent rejects early call

• Added expire_intent_before_deadline_state_unchanged() test
• Verifies rejection with Error::DeadlineNotReached when called before deadline
• Confirms intent state remains Open after failed expiry attempt

#81 — Add test for get_stats reflecting cumulative totals

• Added get_stats_reflects_cumulative_totals_across_multiple_fills() test
• Tests multiple submit/accept/fill cycles to verify counter accumulation
• Includes cancelled and expired intents to confirm they affect TotalIntents but not TotalVolume
• Verifies both counters accumulate correctly across multiple operations

#80 — Add mutation-testing CI job

• Added mutants job to .github/workflows/ci.yml
• Runs cargo-mutants to detect inadequately-tested code
• Set as non-blocking (continue-on-error) to avoid CI failures on surviving mutants
• Future iterations can use baseline results to improve test coverage

Test Results

All 57 existing tests pass. New tests verify:

• Intent expiry rejection before deadline
• Protocol stats accumulation across multiple fills
• Cancelled/expired intents don't contribute to volume

Closes #82, Closes #79, Closes #81, Closes #80

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