feat(nsis): add nsis.sourceforge.net

[idleberg]

Summary

• What changed: added manifest for NSIS
• Why: the NSIS compiler is a common tool to compile Windows installers (e.g. used by electron-builder). pkgx provides a no-hassle way to execute it on-demand.

AI Intent

• Prompt or task statement: Help me document the quirks of this manifest, learning from other install manifests such as Homebrew and MacPorts
• Scope of AI-assisted changes: comments and environment variable quirks were added by Claude Code

Risk Assessment

• Risk level: low / medium / high
• Primary risks:
• Compatibility impact: Like the Homebrew and MacPort, this manifest only builds the compiler and uses the precompiled binaries provided by NSIS (and only target x86).

Rollback Plan

• Revert path:
• Forward-fix path (if revert is not possible):

Validation

• Local checks run: Compile built-in example scripts, e.g. bigtest.nsi
• CI checks expected:

Internal/Public Boundary Check

• No internal-only data, private URLs, secrets, or private runbooks were
  added.