Fix overflow bug in Currency.MulWithOverflow

[peterjan]

This PR fixes an overflow bug in Currency.MulWithOverflow. It was passing the carry from the first high-word add into the second add instead of counting it as overflow, so when that carry hit and the second one didn’t, it reported no overflow on a 129-bit product and returned a result off by 2^64. Net effect: Mul would silently wrap instead of panicking.

[claude]

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[Copilot]

Pull request overview

Fixes incorrect overflow detection/handling in Currency.MulWithOverflow, preventing silent wraparound (and incorrect results) for certain 129-bit products.

Changes:

• Corrected the 128-bit multiplication carry handling so the “middle-word” carry is counted as overflow rather than folded into the result.
• Added a targeted regression test case for the 129-bit product scenario.
• Added a changeset documenting the patch-level fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
types/currency.go	Fixes MulWithOverflow carry propagation to correctly report overflow and preserve low 128-bit result.
types/types_test.go	Adds a regression vector and a randomized stress test around MulWithOverflow.
.changeset/fix_overflow_bug_in_currencymulwithoverflow.md	Records the change as a patch-level update via changeset metadata.

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