Add Linux ARM64 vcpkg support

[timkpaine]

Description

Recognize platform.machine() == "aarch64" on Linux and select vcpkg's arm64-linux triplet. This fixes hatch-cpp metadata generation on native Linux ARM64 runners, which previously failed before compilation because no default triplet could be determined.

Adds regression coverage for both explicit triplet validation and automatic Linux ARM64 detection.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated (not applicable)
• Changelog / version bump (follow-up release required)

[github-actions]

Test Results

79 tests  +2   79 ✅ +2   1m 6s ⏱️ ±0s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 70a96e0. ± Comparison against base commit 12bc06b.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.67%. Comparing base (12bc06b) to head (70a96e0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
+ Coverage   94.63%   94.67%   +0.04%     
==========================================
  Files          14       14              
  Lines        1192     1203      +11     
==========================================
+ Hits         1128     1139      +11     
  Misses         64       64              

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