Assess deprecating `isWithdrawable(uint256)` and define withdrawability API semantics

[franrolotti]

In PR #121, isWithdrawable(id) changed semantics from “current scheduled withdrawer can withdraw” to an aggregate signal: “at least one member can currently claim/withdraw (incl. late claims)”. We added isMemberWithdrawable(id, member) and the viewer now uses it for canWithdraw.

Review notes:

• Semantics may be misleading / breaking for consumers expecting current-round-only.
• Current aggregate implementation can be O(N²) worst-case (nested scans), even if view.

Tasks

• Decide: keep isWithdrawable, rename to explicit aggregate (e.g. isAnyMemberWithdrawable), or deprecate/remove in favor of explicit APIs.
• If kept: make semantics explicit in Natspec/docs and reduce worst-case cost (avoid nested full-member loops; consider round-funded flag/index).
• Add/adjust tests.

Acceptance

Clear API semantics (current-round vs member vs aggregate), no misleading naming, and aggregate check not O(N²).