docs(trace): pipeline overview and CLI usage

[hkimw]

Refs #160, #161, #162, #163, #164, #165.

Summary

• Add docs/trace-pipeline.md covering TraceFrame, TraceStream sources, framing v2, TraceManifest, TraceAnalyzer summary fields, and trace CLI usage.
• Wire the new page into the Sphinx docs toctree.
• Keep the overview evidence-gated to fixture and contract coverage only.

Validation

• git diff --check
• cargo test -p pccx-core --test public_claim_guards
• npm ci
• npx tsc --noEmit
• make -C docs html SPHINXBUILD=/tmp/lab-docs-venv/bin/sphinx-build

Notes

• Sphinx build succeeds with existing warnings in unrelated docs pages.
• No hardware-run capture, throughput, timing closure, target execution, or target-readiness claim is made.