fix: expand extras self-refs, drop PEP 517 fallback

[baszalmstra]

After the lockfile-v7 refactor (#5607) pixi install --locked started reporting false drift on a couple of common shapes. This PR fixes the satisfiability check end-to-end and adapts one stale test path along the way.

Changes

test_workspace_variants_separate_work_directories: work directories moved from .pixi/build/work/<pkg>--<hash>/ to .pixi/bld/<pkg>/<workspace_key>/ in the lockfile-v7 refactor. Test now looks in the new location.

Expand pkg[group] self-references in static requires_dist: UV's static database.requires_dist keeps pkg[group] self-references from [project.optional-dependencies] as-is, while build backends like hatchling write the expanded contents into the wheel METADATA at lock time. The two views compare as different, so satisfiability falsely reports the lock-file as out-of-date for any project that composes its extras (e.g. Deltares/Ribasim's all = ["pkg[a]", "pkg[b]"]).

We now run the same expansion ourselves: each pkg[a, b]; <outer marker> is replaced by the raw entries of groups a and b carrying the outer marker. Cycles in the optional-deps graph (e.g. a → b → a) are broken on the path that closes them.

Also dedups the requires-dist diff message and includes extras, so added: [pkg, pkg, pkg] becomes added: [pkg[a], pkg[b]].

Skip PEP 517 build during the satisfiability check: read_local_package_metadata previously fell back to get_or_build_wheel_metadata when uv's static requires_dist declined. That fallback needs the host conda prefix to have Python, but cross-platform satisfiability runs on platforms that may not be in the lock at all. The resulting PythonMissingError panicked through LazyBuildDispatch::interpreter past this path's missing catch_unwind, aborting pixi run / pixi reinstall on hosts whose platform was missing from the lock (e.g. macOS arm64 against a linux-64-only lock).

The function is now restricted to uv's static path. Ok(Some) feeds drift detection (then runs through the self-extras expansion); Ok(None) (uv's "static doesn't apply" classification) trusts the lock; Err surfaces as PlatformUnsat::FailedToReadLocalMetadata to force a re-solve.

Verified for uv 0.9.5 that database.requires_dist itself does not invoke the interpreter.

Verification

• All 131 satisfiability unit tests pass; new unit tests cover the Ribasim-shape expansion, non-self-references, unknown extras, and the cycle-break case (inline insta snapshots).
• Built target/release/pixi from the branch, ran pixi install --locked against Deltares/Ribasim@main (v6 lockfile): exit 0, install completes, no lock-file not up-to-date and no metadata for local package 'ribasim' has changed in the trace. Static-only path is exercised for ribasim, ribasim-api, and ribasim-testmodels per the debug log.

[baszalmstra]

@nichmor The last two commits in this PR touch some of the code you wrote. Would you be able to take a look whether the changes make sense to you?