Harden movement metric aggregation and add export/API consistency tests

[GuillermoDylan]

What changed

1. Public export surface hardening

   • Added a dedicated regression test suite to validate export consistency across:
     • pywib.__all__
     • pywib.core.__all__
     • pywib.utils.__all__

2. Defensive movement metric behavior

   • Updated movement metric aggregators to behave deterministically when traces are incomplete:
     • velocity_metrics
     • acceleration_metrics
     • jerkiness_metrics
   • If df is provided and traces miss the required computed column, traces are recomputed.
   • Added/updated regression tests in movement test suite to cover:
     • recomputation path with incomplete traces + available df
     • expected error path with incomplete traces + df=None
     • precomputed-trace happy path

Validation

• Movement and focused regression tests executed successfully.
• Export-surface tests pass.
• No lint issues in changed files.