Keep previous shader outputs when a compile fails

[botoddly]

Fixes #619.

• Every target compiles into the temporary directory CompileShader already creates, including the Metal and WGSL rewrites. A failed target leaves .generated untouched.
• After every target succeeds, the old metadata is deleted, the outputs are copied into .generated, and the metadata is written last. A build interrupted while copying finds no metadata and compiles again.

Verification

• A new test builds a shader, breaks it so the SPIR-V output is written and then rejected by the draw parameters check, restores the source and builds again. Every file in .generated matches the original build. The test fails without the fix.
• Pixely.SdlangCompiler.Tests (65) pass. The build integration tests pass on Linux; the Windows-only one did not run.
• The test breaks the shader through the draw parameters check rather than a DXC rejection, which exercises the same path.

[github-actions]

📦 Development package Pixely 0.0.111 is available.

Workflow run (attempt 1)

[github-actions]

✅ Development package validation passed on all platforms.

Workflow run (attempt 1)