Add pre-0.7 framework building blocks

[0xShug0]

Summary

Promotes additive framework building blocks from the upcoming 0.7 work so they can be reviewed and reused earlier, without adding the 0.7 model callsites.

Includes:

• LeakyReLU and convolutional feed-forward modules
• extended streaming/causal Conv1d padding modes
• additive packed/cached attention paths
• NeMo Nano codec runtime
• Mimi encoder streaming APIs
• Qwen batched decode/KV cache and compact logits readback helpers
• sampler offset-block support

Scope

This PR intentionally excludes 0.7 model implementations, model specs, server routes, UI changes, path tests, and stale Music3 branch changes that would revert current main fixes.

Existing callsites should keep their previous behavior because the new paths are opt-in/default-off.

Validation

• git diff --check HEAD~1..HEAD
• Source audit against current main to avoid reverting later main fixes in chunking, model_spec, and BigVGAN.

No runtime/path tests were run for this framework-only extraction.