feat(rewards): accrue protocol fee rewards with view, event, and boundary tests

[Baskarayelu]

Summary

The router had no rewards concept — closest existing state is the fee collected per route (compute_route_fee) that's ultimately owed to FeeRecipient. Issues #419-#421 each describe a slice of a rewards model (a view, an event, boundary tests) that presuppose the underlying accrual exists, so this is the minimal accrual this repo's fee model naturally supports, built in one PR since #419's view and #420's event are two sides of the same state:

• DataKey::RewardsAccrued — protocol-wide lifetime sum of fees collected, saturating_add, same pattern as TotalRoutesAllTime/PairVolume
• get_rewards_accrued() — read-only view (Add a read-only view exposing the current rewards state #419)
• rwd_accr event carrying (fee, running_total), emitted only when fee > 0 — a zero-fee route is not a rewards-state change, so it neither writes nor emits (Emit a dedicated event when rewards state changes #420, 'no duplicate emissions')
• Wired into compute_route_fee's existing effects section, right after the TotalRoutesAllTime write

Tests (#421)

Default-zero, accumulation across multiple routes, zero-fee route is a no-op (no write, no event), event payload carries the correct (fee, running_total), and the over-limit boundary: seeding the counter near i128::MAX and confirming a further accrual saturates instead of panicking.

Test output

Same pre-existing main-at-HEAD build breakage as prior PRs in this batch (103 unrelated errors, reproducible via git stash). Confirmed this change adds zero additional errors.

Closes #419
Closes #420
Closes #421