feat(muxed): Add uint64 round-trip test for max uint64 boundary

[GEEKYFOCUS]

Description

This PR adds a focused unit test to validate correct round-trip encoding/decoding
at the uint64 maximum boundary, ensuring serialization safety without buffer
overflow or precision loss.

Problem

ID integers maxing at uint64 (18446744073709551615) run risks of buffer overflow
or precision drop depending on intermediate serialization logic. Without explicit
testing, subtle errors can go undetected.

Solution

Added TestEncodeMuxedUint64MaxRoundtrip that:

1. Uses maxUint64 := ^uint64(0) as the test value
2. Encodes through EncodeMuxed and decodes through DecodeMuxed
3. Validates bit-for-bit correctness with triple verification:
   • Base account matches exactly
   • ID string representation matches exactly
   • Decoded value converts back to uint64 without precision loss

Testing

✅ All tests pass:

• TestEncodeDecodeMuxedIsLosslessForUint64Max
• TestEncodeMuxedWithJS53Boundary
• TestEncodeMuxedJSONUnmarshalWithJS53Boundary
• TestEncodeMuxedUint64MaxRoundtrip (NEW)
• TestRoundtrip_Uint64EdgeCases (comprehensive edge cases)

Impact

• Prevents regressions at uint64 serialization boundaries
• Adds native Go execution validation
• Strengthens safety guarantees for max ID values

Files Changed

• packages/core-go/muxed/muxed_test.go - Added uint64 max round-trip test

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Issue	Title
#78	Implement uint64 round-trip tests in muxed_test.go for max uint64 values	Link to this issue

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[GEEKYFOCUS]

Hello Maintainer.
Kindly review my PR, so the issue can me be closed. Thanks