Flatten dartpy namespace and add snake_case shims

[jslee02]

• Flatten dartpy’s public surface: promote core symbols to dartpy, alias utils as dartpy.io, and keep dartpy.gui as the sole subpackage; legacy deep modules remain importable but now warn and are scheduled for removal in DART 8.
• Add runtime shims for snake_case aliases and legacy module warnings, with env toggles (DARTPY_ENABLE_SNAKE_CASE, DARTPY_WARN_ON_CAMELCASE, DARTPY_ENABLE_LEGACY_MODULES, DARTPY_WARN_ON_LEGACY_MODULES).
• Ensure the pure-Python shims (_naming.py, _layout.py) are copied/installed with the extension and update docs to describe the flattened layout and migration path.

Context: The existing dartpy API mirrored C++ namespaces and camelCase naming. For the DART 7 line we want a more Pythonic surface while keeping one-version compatibility; DART 8 will drop legacy modules/camelCase.

Before/after example:

# Before (deep module + camelCase)
import dartpy.dynamics as dyn
skel = dyn.Skeleton.create()
q = skel.getPositions()

# After (flattened + snake_case)
import dartpy as dart
skel = dart.Skeleton.create()
q = skel.get_positions()
# file parsers live under dartpy.io (alias of utils)
loader = dart.io.DartLoader()

---

Before creating a pull request

• Run pixi run test-all to lint, build, and test your changes (not run here)
• Add unit tests for new functionality (none added yet; runtime shims only)
• Document new methods and classes (docs updated for layout/naming)
• Add Python bindings (dartpy) if applicable (shims wired; C++ bindings unchanged)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.07%. Comparing base (7160371) to head (39fff61).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2259      +/-   ##
==========================================
+ Coverage   61.05%   61.07%   +0.01%     
==========================================
  Files         353      353              
  Lines       32144    32154      +10     
  Branches     4183     4188       +5     
==========================================
+ Hits        19627    19639      +12     
+ Misses      12517    12514       -3     
- Partials        0        1       +1     

Flag	Coverage Δ
unittests	61.07% <ø> (+0.01%)	⬆️

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