fix(release): sign Darwin binaries on macOS

[heimanba]

Root cause

The stable publish job failed while rcodesign 0.29.0 signed Bun's bun-darwin-x64 binary. It panics parsing the Mach-O code signature with an out-of-range slice. This matches upstream apple-platform-rs issue #194.

Fix

• Run the stable and channel binary jobs on macos-15-intel and use Apple's native codesign path for Darwin binaries.
• Install the matching Gitleaks 8.21.2 Darwin x64 asset; zip/tar are available on the macOS runner.
• Update release docs with the known rcodesign failure mode.

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