Add flat Franka asset and backend body-resolution support

[maxkra15]

Description

Add opt-in FRANKA_PANDA_FLAT_CFG and high-PD config while preserving the old Franka symbols, USD paths, and actuator groups with deprecation warnings. Fix direct rigid-body prim selection, frame-transformer resolution for nested bodies, and Newton material randomization with interleaved shapes. No maintained task migrates in this layer.

This is layer 2 of the draft isaac-sim#7829 split, based on layer 1. The published public-S3 flat asset remains an external merge gate; the qualified package is on isaac-dev.

Validation

• Legacy config contract: 1 passed
• Direct prim resolver: 2 passed
• Nested-body frame transformer: PhysX 1 passed; OvPhysX 1 passed, 1 skipped
• Newton body-shape order: 1 passed
• Formatting, generated docs, and changelog checks passed

Type of change

• Bug fix
• New configuration

Release backport

• Backport after upstream merge

Checklist

• Contribution guidelines followed
• Focused tests and formatting passed
• Documentation and changelog updated

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[maxkra15]

Superseded by upstream isaac-sim#8169, targeting develop with the active-release backport selected.