factory: ship feeToSetter two-step rotation for next deploy

[satyakwok]

The README notes:

Internal audit pass shipped 2026-05-07 (1 HIGH feeToSetter two-step rotation patched in source for v2 redeploys; deployed factory unaffected)

This issue tracks the patch landing in source + getting deployed when a v2 factory is cut. Currently the source presumably has a single-step setFeeToSetter() that transfers admin atomically (canonical UniV2 behavior). Two-step rotation = pending-admin pattern (OpenZeppelin Ownable2Step style).

Why

Single-step rotation is a one-mistake-loses-the-protocol risk. Two-step requires the new admin to explicitly acceptOwnership() before the rotation completes, preventing accidental rotation to a non-controlled address.

Scope

• Refactor SentrixV2Factory.feeToSetter to two-step pattern
• Add tests for the pattern (cancel mid-flight, double-call safety, only-pending-admin can accept)
• Document the change for any consumer who polls feeToSetter

Out of scope

• Rotating the live mainnet factory's admin — that's a separate operator decision
• Migrating existing pools to a new factory