docs(spec): v002.1 architecture explainer

[hkimw]

Summary

• add a public v002.1 architecture explainer under docs/spec
• map the ramp across RTL bitstream pre-flight, launcher readiness contract, pccx-lab trace return path, and architecture decision defaults
• cite docs(spec): resolve v002 quantization, K-drain, and DSP baseline pccx-FPGA-NPU-LLM-kv260#80/#82, feat(kv260): add data-only readiness scaffold pccx-llm-launcher#70, and feat(trace): add serial-tty trace source for kv260 runs pccx-lab#162/#163/#165

Claim guard

• v002.1 wording is evidence-gated and describes pre-flight scaffolds, contracts, trace pipeline work, defaults, and open gates
• 20 tok/s is described only as a target
• no KV260 inference, Gemma 3N E4B board-run, throughput achievement, timing closure, bitstream readiness, production readiness, or stable-release claim is made
• forbidden-phrase guard scan passed for the new spec page

Validation

• env PATH=/home/hwkim/Desktop/github/pccxai/pccx/.venv/bin:$PATH make strict
• env PATH=/home/hwkim/Desktop/github/pccxai/pccx/.venv/bin:$PATH make lint
• rg -n "KV260 inference works|Gemma 3N E4B runs on KV260|20 tok/s achieved|timing closed|bitstream ready|production-ready|stable release" docs/spec/v002.1-architecture-explainer.md returned no matches

Refs pccxai/pccx-FPGA-NPU-LLM-kv260#80
Refs pccxai/pccx-FPGA-NPU-LLM-kv260#82
Refs pccxai/pccx-llm-launcher#70
Refs pccxai/pccx-lab#162
Refs pccxai/pccx-lab#163
Refs pccxai/pccx-lab#165

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