docs(spec): gemma 3n e4b path overview

[hkimw]

Summary

• Add docs/spec/gemma-3n-e4b-path.md, a public Gemma 3N E4B software-path explainer.
• Map the current launcher scaffold across W4 weight prep, tokenizer, architecture spec, serial token streaming, and mock end-to-end orchestration.
• Cite the active pccxai/pccx-llm-launcher PR evidence: #83, #84, #87, #88, and #89.

Claim guard

• Follows the task-cited manual/06_PUBLIC_WORDING_AND_CLAIMS.md claim list.
• Status is scoped to software scaffold, contracts, mocks, and pending evidence.
• The 20 tok/s wording is target-only.
• No board execution, real inference, achieved throughput, completed bitstream, or production status claim is made.

Validation

• git diff --check
• env PATH=/home/hwkim/Desktop/github/pccxai/pccx/.venv/bin:$PATH make lint
• env PATH=/home/hwkim/Desktop/github/pccxai/pccx/.venv/bin:$PATH make strict
• public forbidden-phrase guard scan passed for the new spec page

Refs pccxai/pccx-llm-launcher#83
Refs pccxai/pccx-llm-launcher#84
Refs pccxai/pccx-llm-launcher#87
Refs pccxai/pccx-llm-launcher#88
Refs pccxai/pccx-llm-launcher#89

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