fix(evidence): preserve immutable Evidence records

[jhuang-tw]

Summary

append-evidence.ps1 currently writes Evidence records with WriteAllText, so reusing an existing Evidence ID overwrites the original record and then appends a second row to INDEX.md. That conflicts with the repository's Evidence contract, which treats Evidence as immutable observations.

This PR keeps the change bounded to that failure boundary:

• create E-*.md with atomic FileMode.CreateNew semantics instead of overwrite semantics;
• fail clearly when the normalized Evidence path already exists;
• extend the existing smoke path with a regression that verifies a duplicate ID fails while both the original Evidence file and INDEX.md remain byte-for-byte unchanged.

Validation

The existing Windows/Linux routing matrix already runs smoke.ps1, so the regression exercises the real append-evidence.ps1 entrypoint under native Windows PowerShell 5.1 and the Linux PowerShell shim without adding or changing workflow permissions.

[dhicoc]

本地审查通过并合并：

• 审查结论：Evidence 不可变修复合理（CreateNew + 重复 ID 拒绝 + 字节级回归）
• 额外改动 case-init.sh 大小写归一化、ci.yml 覆盖大写 network_profile，边界清晰
• 本地流程：fetch PR → merge 到 main → smoke.ps1 ALL PASS（含 duplicate Evidence ID rejected without mutation）→ push main

已按协作者约定「本地合并 + 测试通过后再上线」处理。