Add per-pair lifetime route counters and cumulative volume tracking

[mikewheeleer]

Implement per-pair route counters and cumulative volume

Description

The router keeps only a single global DataKey::TotalRoutesAllTime counter in src/lib.rs; there is no per-pair breakdown of how many routes a corridor has served or how much volume has flowed through it. Operators and dashboards cannot rank corridors, detect a hot pair, or reconcile per-corridor fee accrual without scraping every route event off-chain. This issue adds on-chain per-pair route counts and cumulative routed volume.

Requirements and context

• Repository scope: StableRoute-Org/Stableroute-contracts only.
• Add DataKey::PairRouteCount(Symbol, Symbol) (u64) and DataKey::PairVolume(Symbol, Symbol) (i128); increment both inside compute_route_fee using saturating_add.
• Add read entrypoints get_pair_route_count and get_pair_volume (both default 0 when absent).
• Extend PairInfo is out of scope — keep these as standalone getters so the existing struct shape stays stable.
• Do not change any existing error codes; bump TTL on the new slots when written.

Suggested execution

• Fork the repo and create a branch
• git checkout -b feature/contracts-32-per-pair-metrics
• Implement changes
  • Write code in: src/lib.rs — the two new DataKey variants, the increments in compute_route_fee, and the getters.
  • Write comprehensive tests in: src/lib.rs #[cfg(test)] mod test — assert count and volume increment per route, stay isolated per pair, and default to 0.
  • Add documentation: document the per-pair metrics in README.md.
  • Include NatSpec-style doc comments (///) on the new entrypoints.
  • Validate security assumptions: saturating arithmetic cannot panic; counters are monotonic; pairs are independent.
• Test and commit

Test and commit

• Run cargo fmt --all -- --check, cargo build, and cargo test.
• Cover edge cases and failure paths: zero routes, multiple routes one pair, two pairs independent, volume near i128::MAX.
• Include the full cargo test output and a short security notes section in the PR description (threat model + mitigations).

Example commit message

feat: add per-pair route counters and cumulative volume tracking

Guidelines

• Minimum 95 percent test coverage for impacted modules.
• Clear, reviewer-focused documentation.
• Timeframe: 96 hours.

Community & contribution rewards

• 💬 Join the StableRoute community on Discord for questions, reviews, and faster merges: https://discord.gg/37aCpusvx
• ⭐ This is a GrantFox OSS / Official Campaign task and may be rewarded. When your PR is merged you'll be prompted to rate the project — if this issue and the maintainers helped you ship, we'd be grateful for a 5-star rating. Clear questions in Discord and tidy, well-tested PRs are the fastest path to a merge and a reward.

[Baskarayelu]

I’d love to be assigned this issue and contribute across the relevant areas, whether it involves smart contracts, UI development, backend logic, design implementation, or overall system integration. I’m comfortable working across the full stack and can adapt to the project’s technical requirements to ensure a smooth and consistent contribution.