resolve remark call IDs from metadata

[mcjkula]

Summary

Resolve System remark call IDs from runtime metadata instead of hardcoding Bittensor call indices.

What changed

• Runtime call IDs: Adds typed remark call IDs to chain state and resolves System.remark / System.remark_with_event from metadata.
• Submission path: Uses the metadata-resolved remark_with_event call ID when signing outbound remarks.
• Reader path: Uses the resolved call IDs when parsing incoming chain remarks.
• Mirror path: Threads the same call ID state through mirror-backed reads.
• Explicit failure: Fails with a clear runtime error when System.remark_with_event is unavailable.
• Coverage: Adds runtime metadata fixtures and tests for non-default call IDs, missing calls, and refresh/submission paths.

Test plan

• cargo test
• GitHub Actions green: fmt, check, clippy, test, coverage
• Codecov reports all modified coverable lines covered

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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