ci(release): reinstate macOS Intel (x86_64) build target on macos-15-intel (#342)

Intel MacBook users had no installable artifact: the release matrix only
built aarch64-apple-darwin, and Rosetta 2 cannot run arm64 apps on Intel
(it only translates the other direction) — the rationale in the old
"Intel dropped" comment was backwards. Refs #279.

- Add a native `macos-15-intel` matrix leg (GitHub's designated x86_64
  migration target after macos-13 retired Dec 2025; standard image,
  supported through Aug 2027) building --target x86_64-apple-darwin
  with app,dmg,updater bundles.
- Existing per-TRIPLE steps already carry x86_64-apple-darwin cases
  (uv sidecar tar.gz, evermeet.cx ffmpeg/ffprobe — x86_64 Mach-O,
  natively correct on Intel), so the leg flows through the same
  Bundle/Build/Smoke/Verify steps untouched.
- The PR #290 signing path applies automatically: ad-hoc seal from
  tauri.conf.json signingIdentity "-", opt-in APPLE_* stable signing,
  and scripts/verify-macos-signing.sh both gated on runner.os == macOS.
- tauri-action includeUpdaterJson merges the new darwin-x86_64 platform
  key into latest.json alongside darwin-aarch64, so Intel installs
  auto-update on both Stable and Preview channels.
- docs/install/macos.md: table telling users which DMG (aarch64 vs x64)
  matches their Mac, and the from-source fallback for old releases.

Co-authored-by: mergetest <test@local>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs/install/macos.md

@@ -35,6 +35,19 @@ Download the latest DMG from the
 [Releases page](https://github.com/debpalash/OmniVoice-Studio/releases/latest),
 double-click to mount, drag **OmniVoice Studio.app** into `/Applications`.
 
+Pick the DMG that matches your Mac (check **Apple menu → About This Mac → Chip/Processor**):
+
+| Mac | DMG to download |
+|-----|-----------------|
+| Apple Silicon (M1/M2/M3/M4…) | `OmniVoice.Studio_<version>_aarch64.dmg` |
+| Intel | `OmniVoice.Studio_<version>_x64.dmg` |
+
+The architectures are **not** interchangeable: an Intel Mac cannot run the
+`aarch64` build (Rosetta 2 only translates the other direction — it lets Apple
+Silicon run Intel apps, never the reverse). If a release predates the Intel
+build target and has no `x64` DMG, use the
+[install-from-source path](#install-from-source) above instead.
+
 If the first launch is blocked by macOS Gatekeeper ("OmniVoice Studio cannot be
 opened because the developer cannot be verified"), see the next section — it
 opens with one right-click, no Terminal.