Preserve split package metadata in sdists

[timkpaine]

Description

Split sdists currently rebuild as the base package because Hatchling resolves dynamic dependencies from PKG-INFO and skips metadata hooks. Register an sdist build hook that packages pyproject.toml with the generated split-package name so downstream wheels retain the selected extra and dependency.

Document the required sdist hook configuration and add an end-to-end sdist-to-wheel regression test.

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 (if applicable)
• Changelog / version bump (not applicable)

[github-actions]

Test Results

4 tests  +1   4 ✅ +1   3s ⏱️ -1s
1 suites ±0   0 💤 ±0 
1 files   ±0   0 ❌ ±0 

Results for commit d8c8ba6. ± Comparison against base commit 43f3660.

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.15%. Comparing base (43f3660) to head (d8c8ba6).

Files with missing lines	Patch %	Lines
hatch_multi/plugin.py	28.94%	27 Missing ⚠️
hatch_multi/hooks.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #47       +/-   ##
===========================================
- Coverage   82.20%   72.15%   -10.05%     
===========================================
  Files           7        7               
  Lines         118      176       +58     
===========================================
+ Hits           97      127       +30     
- Misses         21       49       +28     

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