Restore reproducible native and PE artifact build CI

[stakach]

Migrated roadmap scope. Historical context: completion ledger.

Dependencies: #4.

Acceptance

• Pin supported nightly and update custom-target ABI fields deliberately; diagnose current removed allows-weak-linkage target field.
• Provision only required public source inputs and verify generated PE/native artifacts.
• Run build lanes separately from host specs with clear ownership; these are not desktop or E2E proof.

Track progress and verification here and in PRs. Host specs, native builds and end-to-end proof are distinct; retire superseded machinery at its actual cutover.