Canonicalize resolved extra availability in lockfiles

[zaniebot]

A universal lock can retain a requested extra after all of that extra's resolved dependencies have been ruled out by markers. The serializer omits the empty section, so reading and refreshing the just-written lock removes the incoming extra reference. Drop empty extra sections after assembling the resolver output, then normalize incoming extra references before sorting and validating dependency edges. Existing locks may use an explicitly empty section to activate marked dependencies, so the reader retains those sections. Declaration-level provides-extras metadata remains available, and extras with reachable dependencies in another marker domain remain in the lock.

[codspeed-hq]

Merging this PR will not alter performance

✅ 25 untouched benchmarks

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_{Comparing zb/future-20260912-resolver-28 (4276c18) with zb/future-20260912-resolver-27 (e1b348d)}