CI: platform build matrix red from runner-image drift (macos-26/arm/windows)

[ww2283]

Context

When PR #2's lint/ty gates went green, the full build matrix ran and 23/27 platform build jobs failed (macos-26, macos-14/15, macos-15-intel, ubuntu-22.04-arm, windows-2022 — C++ builds of the faiss/DiskANN submodules). Run: https://github.com/ww2283/LEANN/actions/runs/32278844326

PR #2 was pure Python (cli.py, sync.py, tests) and did not touch submodule pins or build config, so these failures are not from the PR. Fork main's last full-matrix green was 2026-07-04 (run 28711105728); the failing set includes macos-26, a runner image that did not exist then — this is runner-image/toolchain drift.

Impact

• Any merge gated on checks-green is blocked (PR Multi-invocation incremental builds: --sync-key, changes/verify subcommands, zero-chunk snapshot commit, LEANN_NO_REGISTER #2 was merged by explicit human override with nightshift off).
• Log inspection was cut short by GitHub API rate limiting; the concrete compile errors per platform still need triage.

Acceptance

• Build matrix green on main, or failing images explicitly pinned/removed from the matrix with rationale.