fix(nonce): use exponential backoff in safeAcquire

[wildanniam]

Summary

• change NonceManager.safeAcquire from linear retry delays to exponential backoff (delayMs * 2 ** attempt)
• align the parameter documentation with the implemented behavior
• add a deterministic fake-timer regression test covering the exact 100, 200, 400 backoff sequence

Verification

• npm test — 74 test files passed, 1 skipped; 964 tests passed, 2 skipped
• npm run typecheck
• npm run lint
• npm run build
• focused nonce/locking run — 4 test files and 28 tests passed
• changed-file ESLint and git diff --check

Baseline note

Initial verification against the previous main commit (3bdb66d) exposed 18 unrelated stale test failures, four existing typecheck errors, and two existing lint errors. Those results reproduced on a clean worktree and were not introduced by this change.

Before opening this PR, main received a020f5a (test(ci): fix lint, typecheck and stale tests on main). This branch was rebased onto that commit and the complete test, typecheck, lint, and build verification is now green.

Risks / Notes

• no public API or method-signature changes
• jitter and WebSocketRelayer behavior remain outside this focused issue
• the existing single-waiter queue and lock handoff behavior are unchanged and covered by the related nonce tests

Closes #551

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