#148 Fix Windows npm self-upgrade spawning

Summary

• share the existing Windows-safe child-process resolver across setup and upgrade
• run npm upgrades through node/npm-cli.js instead of an unspawnable npm.cmd shim
• resolve the installed insta shim safely when verifying the upgraded version

Verification

• npm run typecheck
• 28 upgrade resolver tests
• 5 existing resolveSpawnable tests
• Windows build-output smoke tests for npm --version and insta --version

Windows regression

Before this change, insta upgrade failed with spawn npm ENOENT. Node also rejects direct npm.cmd spawning with EINVAL. The resolved node entry point now exits 0 on Windows.

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Summary by cubic

Fixes Windows npm self-upgrade spawn failures so insta upgrade no longer errors with ENOENT or EINVAL on Windows.

Bug Fixes

• Moves the Windows-safe resolver into a shared src/spawn.ts used by both setup and upgrade.
• Runs npm upgrades through node/npm-cli.js instead of the unspawnable npm.cmd shim.
• Resolves the installed insta shim safely when verifying the upgraded version.
• Launches .cmd shims via an absolute System32 cmd.exe (resolved from SYSTEMROOT) and skips the wrapper when a PATH directory or shim path contains shell metacharacters.
• Looks up PATH/PATHEXT case-insensitively, so the version probe still resolves shims through copied environments that keep Windows' Path casing.