docs(spec-runner): document BigInt.parse() rationale for uint64 muxed…

[Sendi0011]

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Title: docs(spec-runner): document BigInt.parse() rationale for uint64 muxed IDs

Body:

Closes #188

## What changed
Added inline comments in `packages/core-dart/test/spec_runner_test.dart`
at both BigInt.parse() call sites inside the spec vector loop (muxed_encode
and muxed_decode cases).

## Why
Stellar muxed IDs are unsigned 64-bit integers (uint64), valid range
0..2^64-1 (18446744073709551615). Two silent failure modes exist without BigInt:

- Dart's native `int` is signed 64-bit — values above 2^63-1 overflow silently.
- JSON number parsing loses precision above 2^53 (JavaScript's safe-integer
  boundary), which is why the spec vectors encode IDs as strings.

BigInt.parse() is the only correct way to ingest these values across the full
uint64 range without truncation or corruption, which is critical for
cross-platform interoperability.

## Testing
No logic changed — documentation only. Existing spec runner tests continue
to pass unchanged.

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