native: promote macos-15-intel qualification lane into the release matrix

[qnbs]

Context

Part of the post-#477 reconstruction reconciliation program's S5 slice (Intel macOS qualification lane).

macos-13 (the last GitHub-hosted Intel runner) was removed from tauri-build.yml's production matrix on 2026-07-28 after repeatedly failing to provision (queue-hung, not a build failure) — see docs/TAURI-CI.md for the incident. GitHub's tauri-build.yml comment referenced this as "a re-opened follow-up" but no issue previously tracked it; this issue is that tracking artifact.

What's done

• Empirically confirmed (2026-08-26) that GitHub's replacement label macos-15-intel (available since 2025-09-18, through August 2027) schedules and runs successfully on this org's plan.
• New qualification-only workflow: .github/workflows/tauri-intel-qualification.yml — workflow_dispatch only, permissions: contents: read, no signing secrets, no Release/latest.json mutation. Builds the app on macos-15-intel as a real Tauri build to keep proving the toolchain still works there.
• Documented current status in docs/native/INTEL-MACOS-QUALIFICATION.md.

What's open before promoting Intel back into the production release matrix

1. Soak/repeat testing — one successful qualification run doesn't rule out the same queue-availability issue that removed macos-13; needs repeated real-world observation over time, not a single manual dispatch.
2. Artifact parity check against the existing macos-latest (Apple Silicon) bundle.
3. Ongoing maintenance-cost decision — doubling macOS CI minutes per release for the current Intel-Mac user base is a product call, not purely technical.
4. Explicit promotion PR once (1)-(3) are resolved: add macos-15-intel to tauri-build.yml's bundle matrix, add a darwin-x86_64 entry to the latest.json generator's expected-platform set, update docs/TAURI-CI.md's Build matrix table.

Not blocking any other work; this is deliberately kept as a separate, opt-in qualification lane.