fix(p2ms): isAcceptableSignature always returned true due to operator precedence

[Grizouforever]

Summary

In ts_src/payments/p2ms.ts, the isAcceptableSignature function contains a logic bug caused by JS operator precedence:

// Before (buggy)
(opts!.allowIncomplete && (x as number) === OPS.OP_0) !== undefined

The !== undefined check is applied to the result of the && expression, which is always a boolean (true or false). Since neither true nor false is undefined, this condition always evaluates to true — meaning isAcceptableSignature accepted any input as a valid signature, completely bypassing P2MS signature validation.

Fix: Coerce the expression to a proper boolean with !!:

// After (fixed)
!!(opts!.allowIncomplete && (x as number) === OPS.OP_0)

Impact

Without this fix, P2MS inputs with invalid/garbage signatures pass validation silently. The function was intended to only allow OP_0 placeholders when allowIncomplete is set.

Tests

All 2660 existing tests pass, including the p2ms-specific tests that assert "throws Input has invalid signature(s)". No test changes were needed — the existing fixtures already encode the correct expected behavior.

p2ms
  ✔ throws Input has invalid signature(s)
  ✔ p2ms, m derives from ["output"]
  ...
  ✔ throws Input has invalid signature
  ✔ throws Input has invalid signature
  ✔ throws Input has invalid signature
  ✔ throws Witness has invalid signature

2660 passing (11s)