Fix Windows ACP process launch

[SivanCola]

Summary

• resolve Windows where reasonix results without selecting extensionless npm shell shims
• prefer the packaged native x64/arm64 executable and safely support configured .cmd/.bat wrappers
• add Windows Extension Host coverage for explicit binary and PATH modes, and gate releases on it

Root cause

v0.3.0 spawned the PATH/configured npm shim directly with the Node child process API. Windows cannot directly execute .cmd/.bat wrappers without command interpretation, producing EINVAL; selecting the extensionless npm shell shim produced ENOENT.

Validation

• npm run lint
• npm test (65 tests)
• npm run test:vscode
• npm run smoke:acp
• npm audit --audit-level=high
• npm run package:vsix
• npm run verify:vsix-contents
• Parallels Windows 11 ARM64: explicit reasonix.cmd and PATH modes passed; the real Reasonix 1.19.4 shim resolved to the packaged ARM64 executable and executed; ACP initialize passed (the clean VM still needs a configured model for session/new)

Closes #3