Feature: Auto-share default with compatibility-aware grouping

[cvanelteren]

Summary

This PR changes default subplot sharing to a compatibility-aware share='auto' mode, while preserving explicit share levels (0..4, aliases) as hard overrides.

The goal is to avoid noisy/inaccurate default sharing for mixed axis families (for example cartesian + polar) and to keep expected shared behavior for compatible grids.

Design Changes

• Added parsing for share='auto' in Figure (sharex/sharey/share).
• Added compatibility checks for auto-sharing groups:
  • axis family/projection compatibility (cartesian, polar, geo, external containers)
  • scale compatibility (xscale / yscale)
  • unit/converter domain compatibility
• Reworked sharing group formation to partition row/column candidates into compatible subgroups.
• Kept explicit sharing semantics (False/0, labels/1, limits/2, True/3, all/4):
  • explicit modes still attempt sharing and can warn on incompatibility.
• Added auto-refresh flow for scale changes:
  • in auto mode, scale changes can unshare/re-evaluate groups.
  • after re-grouping, shared groups are autoscaled from sibling data to keep limits consistent.
• Refactored repeated join/autoscale logic into figure helpers to reduce duplication.

Behavioral Expectations

• Default behavior (subplots.share='auto'):
  • compatible axes share by default.
  • incompatible mixes are left unshared without warnings.
• Explicit behavior (share='all', etc.):
  • sharing is forced where requested.
  • incompatible axes emit warning(s) and are skipped.
• Local axis intent in auto mode:
  • formatting one axis to a different scale can split that axis from its previous share group.
• Group intent in auto mode:
  • formatting a whole subplot grid preserves sharing and recomputes shared limits from all siblings.

Tests Added

• default share mode is auto
• auto mode skips mixed cartesian/polar without warning
• explicit sharing warns for mixed cartesian/polar
• auto mode local y-scale change splits group
• explicit sharey propagates scale changes
• auto mode grid y-scale change keeps shared limits
• auto mode mixed x unit domains remain unshared after refresh

Docs Updated

• API docstring for sharex/sharey/share now documents 'auto' and explicit override semantics.
• Subplots user guide now documents all explicit levels plus 'auto' and includes a mixed cartesian/polar example.

Out of Scope

• Full delegation-first share lifecycle redesign.
• New user-facing API for compatibility policy customization.