feat(contracts): add ballot participation snapshot and tiebreak-windo…

[Nanle-code]

…w accessor

Closes #696

• ballot_participation_snapshot(round_id): returns eligible_voters, votes_cast, participation_bps (votes*10000/eligible, 0 when unconfigured), round_active
• tiebreak_window(round_id): returns window_start, window_end, tiebreak_required, window_open (live, computed at read time)
• Unknown round ids return exists=false with zeroed fields
• 8 unit tests: success, full participation, zero eligible, missing, window open/closed/not-required

Description

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Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

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Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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📥 Commits

Reviewing files that changed from the base of the PR and between 760aa52 and 1bf1beb.

📒 Files selected for processing (6)

• contracts/Cargo.toml
• contracts/map-voting/Cargo.toml
• contracts/map-voting/src/lib.rs
• contracts/map-voting/src/storage.rs
• contracts/map-voting/src/test.rs
• contracts/map-voting/src/types.rs

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