feat(bw): implement percentage-based rollout

[limitofzero]

Summary

Partial rollout gate for the balances-watcher SSE stack. The previous all-or-nothing isBwEnabled boolean flag is replaced by a numeric bwEnabledPercentage in LaunchDarkly, so we can shift the watcher onto a limited share of wallets and grow the cohort as barn stability holds.

Gating is deterministic per wallet — BigInt(account) % 100n < percentage — so the same address stays in the same bucket across refreshes/tabs, no flip-flopping between watcher and multicall polling.

To Test

Prereq: in LaunchDarkly enable the bwEnabledPercentage flag (type: number). The old isBwEnabled boolean is no longer read and can be retired.

1. Full rollout — bwEnabledPercentage = 100, connect a wallet, open swap

• DevTools Network shows POST /sessions/* and GET /sse/*/balances/* — watcher is running
• Multicall stack (BalancesAndAllowancesUpdater / PriorityTokensUpdater) is NOT mounted while the watcher is Healthy

2. Off — bwEnabledPercentage = 0, refresh

• No /sessions / no SSE requests
• Multicall polling is the only balances source

3. Out-of-bucket wallet — bwEnabledPercentage = 50

• Connect a wallet whose address bucket >= 50 (e.g. 0x…63 → bucket 99, since 0x63 = 99)
• Watcher is NOT running; multicall stack is running

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✨ Finishing Touches

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• Commit unit tests in branch feat/implement-percentage-based-rollour-bw

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