fix(resources): validate required executables

[Luyoung0001]

Context

This is remediation 5 of 6 extracted from the code review follow-up in #182. It addresses incomplete resource health checks found during review of f5e9af0f536878f804ef51887e5f5ab734388a24.

Finding

A resource could be reported as installed when its marker path merely existed. Directories, non-executable files, or incomplete staged archives could therefore pass health checks and fail only when the tool was launched.

Changes

• Require executable markers to be ordinary files.
• Check execute permission on POSIX and file accessibility on Windows.
• Validate required markers for ECC-FE, Verilator, and the RISC-V toolchain.
• Apply the same validation to staged archives, installed-resource health, UI status, and runtime path resolution.
• Preserve the previous healthy installation and manifest when replacement validation fails.
• Add regression coverage for directory markers and missing executable permissions.

Result

Resource Manager now reports Installed only when the required tool entry points are present and usable. Broken replacement archives fail before activation without destroying a working installation.

Verification

• Cherry-picked independently onto the latest main.
• Complete pnpm run check passed on this single-fix branch.
• Resource refresh was manually verified to keep healthy resources in the Installed state.

Review lineage

• Combined review follow-up: fix(gui): address six frontend workflow code review findings #182
• Original remediation commit: 7ec485e
• Independent branch commit: e66e158

[Emin017]

The previous canonical-policy duplication has been fixed by the new focused module. One blocking marker-type gap remains: non-executable markers still use existence-only validation, so incomplete archives can pass the same staged and installed health boundary this PR is intended to harden.

[Emin017]

The marker-type behavior from the previous review is fixed. One test-quality blocker remains.