ci: exercise shell workflows on macOS system Bash

[jhuang-tw]

Summary

The repository documents and supports macOS shell workflows, but the existing CI executes Bash compatibility checks only on Ubuntu. A recent macOS fix (#64) showed why that gap matters: Bash 4-only ${var,,} syntax passed the Linux path but fails under the system Bash shipped on macOS.

This PR adds one focused macOS compatibility workflow without changing product behavior:

• run only when tracked shell scripts or this workflow change;
• use /bin/bash explicitly so the checks exercise the macOS system shell rather than a newer Homebrew Bash;
• parse every tracked .sh file with /bin/bash -n;
• execute one structured-router path;
• execute the default authorized case-init/case-guard path;
• execute the uppercase network-profile regression that previously exposed the Bash 3.2 incompatibility;
• keep permissions read-only and cap the job at five minutes.

Why

Ubuntu CI is useful for shell correctness, but it cannot prove compatibility with the older system Bash used by macOS. This keeps that platform contract executable so future shell changes do not silently reintroduce the same class of failure.

Scope

• CI only
• one new workflow file
• no source changes
• no dependencies
• no route-rule changes
• no overlap with the full Bash routing benchmark in test(routing): run full benchmark through Bash router #69

[dhicoc]

本地合并流程已完成：

1. 审查 diff（范围与动机成立）
2. git merge --no-ff 到本地 main（顺序 test(routing): run full benchmark through Bash router #69 → ci: exercise shell workflows on macOS system Bash #70 → fix(docs): clarify report flavor routing and evidence traceability #71）
3. skills/scripts/smoke.ps1 → ALL PASS
4. skills/scripts/verify-routing-coherence.ps1 → ALL PASS（含 fix(docs): clarify report flavor routing and evidence traceability #71 新增 vendor 规则断言）
5. push main

说明：本机 Windows 无系统 Bash，test-routing.sh 全量 benchmark 以 PR CI（Ubuntu）与 #69 设计为准；#70 的 macOS system Bash job 已在 PR CI 通过。