feat(config): flip .backend = .sdl to the extracted package by default (#386)

[apotema]

builtinProvider(.sdl) → labelle-sdl 0.1.0 — 4th default-external backend. Desktop-only, loop-style; inherits the cache fetch-path + splice from the prior flips. labelle-sdl published + CI-green. Test rework: enum-shorthand + the two sdl build_zig tests (local pkg + project_dir). zig build test green.

Summary by CodeRabbit

• New Features

  • SDL projects now resolve as an external backend when no custom backend package is set.
  • Build generation now supports pointing SDL to a local in-repo backend package.

• Bug Fixes

  • Corrected backend detection so SDL is treated consistently with other external backends.
  • Updated related checks to report SDL as external and use the expected backend package resolution.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: a1d8bd25-6c6a-40d7-9203-83edb6bbc8ce

📥 Commits

Reviewing files that changed from the base of the PR and between 2e93fb8 and f334bbd.

📒 Files selected for processing (3)

• src/backend_registry.zig
• src/config.zig
• test/build_zig_tests.zig

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📝 Walkthrough

Walkthrough

This PR changes how the .sdl built-in backend tag is resolved: ProjectConfig.builtinProvider() now maps .sdl to an external labelle-sdl package instead of treating it as bundled. Corresponding test expectations in backend_registry and build_zig_tests are updated to reflect external resolution.

Changes

SDL External Backend Resolution

Layer / File(s)	Summary
Provider mapping for .sdl backend src/config.zig, src/backend_registry.zig	builtinProvider() maps .sdl to external package labelle-sdl v0.1.0 instead of returning null; backend registry test now expects .sdl to be external with a non-null effective package.
Build generation tests for SDL package wiring test/build_zig_tests.zig	Two SDL build-config tests now explicitly set backend_package to local:backends/sdl and project_dir = "." in generator options.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• labelle-toolkit/labelle-assembler#398: Introduces external backend handling via the same isExternal()/provider resolution path this PR extends to .sdl.
• labelle-toolkit/labelle-assembler#419: Modifies the same builtinProvider()/backend_registry logic to make bgfx external by default, mirroring this PR's treatment of .sdl.
• labelle-toolkit/labelle-assembler#431: Same mechanism applied to .null → labelle-null, touching identical files and patterns.

Poem

🐰 Little SDL hopped outside today,
No longer bundled, free to play,
A package fetched from far away,
"local:backends/sdl" shows the way,
Hooray, hooray, external SDL! 🎉

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly states the main change: making .backend = .sdl use the extracted package by default.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/386-flip-sdl-default

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[gemini-code-assist]

Code Review

This pull request extracts the sdl backend into an external package instead of keeping it bundled. It updates the backend registry tests, configures the sdl repository details in ProjectConfig, and updates the build tests to reference the external sdl package. There are no review comments, so no feedback is provided.

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