build: D1 cross-compile via cargo-zigbuild (4 non-macOS targets)

Replace the single-target musl cc with cargo-zigbuild + zig in the flake
devShell — one universal cross cc/linker (incl. rusqlite's bundled SQLite
C) for all four non-macOS D1 targets, added to rust-toolchain.toml:
  x86_64/aarch64-unknown-linux-musl              (static, D2)
  x86_64-pc-windows-gnu, aarch64-pc-windows-gnullvm  (standalone .exe)

Windows links -lsynchronization (std WaitOnAddress), which rust-overlay's
toolchain and zig's mingw don't ship; the symbols are forwarded by
kernel32, so an empty stub libsynchronization.a (ci/winstub/, wired via
.cargo/config.toml for the windows targets only) satisfies the linker.
Verified: all four build; linux static; windows valid PE32+.

.cargo/config.toml

@@ -0,0 +1,17 @@
+# Windows cross-link fix for the D1 release matrix (cargo-zigbuild).
+#
+# Rust's std links `-lsynchronization` on Windows (WaitOnAddress-based thread
+# parking). Rust normally satisfies this from the `self-contained` mingw libs
+# of its `rust-mingw` component — which rust-overlay does NOT ship — and Zig's
+# bundled mingw (used by `cargo zigbuild`) doesn't provide `libsynchronization.a`
+# either. The actual symbols are *forwarded by kernel32* (already linked), so an
+# empty stub import lib is enough to satisfy the linker. See `ci/winstub/`.
+#
+# These sections apply ONLY when building for the Windows targets, so host
+# builds (the gate's `cargo test`/`clippy`) and the Linux release targets are
+# unaffected.
+[target.x86_64-pc-windows-gnu]
+rustflags = ["-L", "native=ci/winstub"]
+
+[target.aarch64-pc-windows-gnullvm]
+rustflags = ["-L", "native=ci/winstub"]