feat(repo): name the macOS app bundle and document the Gatekeeper step

[sepehr-safari]

Summary

Name the packaged macOS app bundle OCPP DebugKit Studio.app (today it is
studio.app, only displayed as the long name), and document the one-time macOS
Gatekeeper step for the ad-hoc (un-notarized) build directly — since Homebrew is
not a viable channel for an un-notarized cask (see #81).

Background

#81 (Homebrew cask) is closed not-planned: Homebrew is removing --no-quarantine
and will end support for casks that fail Gatekeeper checks on 2026-09-01
(Homebrew/brew#20755), so a cask for an ad-hoc build is a dead end. Notarization —
the only way to remove the Gatekeeper prompt entirely — is a deliberate post-0.5
non-goal for now. So we document the manual un-quarantine step for a technical
audience, and make the app's on-disk name match what users see so that step reads
naturally.

Scope

• Rename the macOS bundle to OCPP DebugKit Studio.app via
  native package --output "…/OCPP DebugKit Studio.app" in the release workflow.
  The CLI/executable name stays studio (Contents/MacOS/studio); the DMG asset
  name is unchanged (studio-<version>-macos-ReleaseFast.dmg).
• Rewrite the README macOS install: direct download + drag to Applications,
  then clear Gatekeeper once with
  xattr -dr com.apple.quarantine "/Applications/OCPP DebugKit Studio.app"
  (Terminal) or System Settings → Privacy & Security → Open Anyway (no
  Terminal). Note that ad-hoc signing is what makes this sufficient on Apple
  silicon (unsigned arm64 code will not run even unquarantined).
• Update RELEASING.md (local-verify path + Gatekeeper note) and CHANGELOG.
• Ships in v0.5.1 so the published app matches the docs.

Acceptance criteria

• native package --target macos --output "…/OCPP DebugKit Studio.app" --archive
  produces a bundle named OCPP DebugKit Studio.app, and the DMG contains it.
• README documents the download + un-quarantine flow accurately.
• v0.5.1 is published; the release DMG mounts to OCPP DebugKit Studio.app.

Out of scope

• Notarization; a universal arm64 + x86_64 build; any package manager.