Refactor atomic action target contracts

[yuecideng]

Description

This PR refactors atomic-action target and runtime-state contracts to make ownership and extension boundaries explicit.

• Keep only framework-level target abstractions in core.py and colocate action-specific target dataclasses with their primitive implementations.
• Add the open ActionTarget marker and a neutral ObjectActionTarget(semantics) base for object-centric actions without introducing a generic, ambiguous pose field.
• Re-export concrete targets from the package root and expose BuiltinTarget as the closed union of built-in executable target contracts.
• Split action-specific contracts such as PlaceTarget and PressTarget; keep CoordinatedPickmentTarget as a compatibility alias for CoordinatedPickTarget.
• Track held objects in WorldState by control part, separating runtime held-object state from desired action targets.
• Update tutorials, benchmarks, tests, public documentation, and agent context for the new APIs.
• Add a dedicated atomic_actions.targets API-reference page and include concrete Target contracts in the primitives autosummary, following the module documentation structure introduced by Complete API reference module descriptions and stale listings #445.

The target hierarchy is motivated in part by the shared object-semantics requirements surfaced in #432: actions can share the minimal semantic-object contract while retaining precise action-specific pose roles.

The branch is rebased onto origin/main at 21091106 (including #441 and #445).

Dependencies: None.

Related: #432

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves an existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Migration notes

• Import concrete targets from embodichain.lab.sim.atomic_actions, not atomic_actions.core.
• Use PlaceTarget for Place and PressTarget for Press.
• Use semantics= when constructing CoordinatedPickTarget.
• Read and update held objects through WorldState.held_objects and WorldState.coordinated_held_objects, keyed by control part.

Validation

• black --check --diff --color ./ — 531 files unchanged.
• pytest -q tests/sim/atomic_actions — 173 passed, 1 skipped.
• Pickup and coordinated-pickment tutorial CLI smoke checks passed after the rebase.
• Coordinated-pickment headless CPU tutorial completed successfully with a (1, 96, 16) trajectory.
• Sphinx HTML documentation build completed successfully; the generated Target API page contains ObjectActionTarget, and existing repository warnings remain.
• 32 changed Python files passed header, future-import, __all__, and diff-whitespace checks.

Screenshots

Not applicable; this is an API and architecture refactor.

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation.
• I have added tests that prove my change is effective.
• Dependencies have been updated, if applicable.

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