Add PiSSA initialization for Megatron LoRA adapters

PiSSA (https://arxiv.org/abs/2404.02948) seeds LoRA A/B from the principal
singular components of the base weight and freezes the residual W_res = W - AB
in place of the full W, so the adapter starts on the principal subspace instead
of at zero.

- pissa_init.py: pure, CPU-testable pissa_decompose() + TP-aware apply_pissa_init()
  that gathers each base weight across the TP group, SVD-decomposes, and writes
  the residual base + A/B back in megatron-bridge's sharded layout.
- SkyRLLoraConfig.pissa flag; applied in the megatron worker's lora pre-wrap hook
  (after HF weights load, before register_pristine), so no megatron-bridge edits.
- CPU unit tests for the decomposition math (reconstruction, principal
  correctness, alpha/rank scale invariance).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: kalectory