Implement Encoding for Int serde support

[tob-joe]

Implement Encoding for Int<LIMBS>.

Int already has Serialize and Deserialize impls behind the serde feature, but those impls require Int<LIMBS>: Encoding. Since Int did not implement Encoding, the serde impls were not reachable for signed fixed-width integer aliases like I256.

This implements Encoding for Int<LIMBS> by delegating to the wrapped Uint<LIMBS> representation. That preserves the existing two's-complement byte layout used by signed integers.

The tests add a dedicated serde trait coverage matrix for the crate's explicit serde impl surfaces, including representative fixed-width unsigned and signed integers, invariant wrappers, arithmetic wrappers, boxed integers under alloc, and ConstMontyForm. The tests also exercise I256 deserialization through serdect without adding a concrete serializer dev-dependency.

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This work was completed by Trail of Bits as part of the Patch The Planet project in collaboration with OpenAI. The vulnerability was identified primarily by the Codex coding agent, and manually reviewed before submission.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.10%. Comparing base (3f7dede) to head (76e854e).

Files with missing lines	Patch %	Lines
src/int/encoding.rs	50.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1286      +/-   ##
==========================================
+ Coverage   91.01%   91.10%   +0.09%     
==========================================
  Files         189      189              
  Lines       22160    22172      +12     
==========================================
+ Hits        20169    20200      +31     
+ Misses       1991     1972      -19     

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