🔥 Drop WARPKIT_DEV escape hatch in favor of cmake build cache

[vanandrew]

Summary

• Remove the WARPKIT_DEV=1 branch in warpkit/init.py — it pointed import warpkit at a manual cmake -B build build to skirt slow uv sync rebuilds; FETCHCONTENT_BASE_DIR + ccache does the same job without forking the import path.
• Document the cache env vars in CLAUDE.md under a new "Speeding up uv sync rebuilds" subsection.

Why

uv sync runs the build backend in a fresh tempdir each time, so CMake re-fetches and re-builds ITK from scratch. WARPKIT_DEV worked around that by routing the import through a long-lived cmake -B build directory — but it required users to remember to set the env var, and the fallback path made __init__.py carry two import strategies.

Setting these once in your shell:

export FETCHCONTENT_BASE_DIR=$HOME/.cache/warpkit-fetchcontent
export CMAKE_CXX_COMPILER_LAUNCHER=ccache

— gets the same speed-up. ITK's tarball gets fetched + extracted once total; ccache hits on rebuilds of the same ITK sources in a fresh build dir. CMake still re-configures per sync (PEP-517 doesn't expose a persistent build dir), but the heavy work is amortized.

WARPKIT_DEV wasn't documented anywhere outside the code, so removal is a clean op.

Test plan

• uv sync --group dev --config-setting editable_mode=strict builds successfully without WARPKIT_DEV
• Verify FETCHCONTENT_BASE_DIR survives across syncs (manual: ls ~/.cache/warpkit-fetchcontent populated after first sync)
• Verify ccache hits on second sync (ccache -s shows non-zero hits after second sync)
• CI green

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR removes the undocumented WARPKIT_DEV import-path escape hatch and replaces that workflow guidance with documented CMake caching/ccache environment variables to speed up repeated uv sync rebuilds.

Changes:

• Simplified warpkit/__init__.py to always import the packaged warpkit_cpp extension (dropping the repo-root build/ import path).
• Added documentation to CLAUDE.md describing FETCHCONTENT_BASE_DIR and CMAKE_CXX_COMPILER_LAUNCHER=ccache to amortize ITK fetch/build cost across syncs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
warpkit/__init__.py	Removes WARPKIT_DEV conditional import logic; always imports warpkit_cpp from the package.
CLAUDE.md	Documents environment-variable-based caching approach for faster uv sync rebuilds.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.14%. Comparing base (aceacc9) to head (1204372).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
+ Coverage   94.44%   95.14%   +0.69%     
==========================================
  Files          15       15              
  Lines        1081     1071      -10     
==========================================
- Hits         1021     1019       -2     
+ Misses         60       52       -8     

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