Add Sparkle auto-updates and Sparkle-free Mac App Store target

[austin-smith]

What Changed

Sparkle auto-updates for the direct-download (GitHub) build:

• Sparkle 2.9.4 via SPM, linked to the app target with a macOS platform filter (never reaches iOS builds).
• All Sparkle surface lives in UpdaterController.swift behind #if os(macOS) && canImport(Sparkle): Check for Updates… in the app menu, an auto-check toggle in Settings → General.
• Sandbox-compliant setup: Installer XPC service enabled, macOS-only entitlements file with the network-client entitlement and Sparkle's mach-lookup exceptions, wired via CODE_SIGN_ENTITLEMENTS[sdk=macosx*].
• SUFeedURL points at GitHub Pages; SUPublicEDKey embedded (private key lives in the repo secret SPARKLE_ED_PRIVATE_KEY).
• Release workflow gains two jobs after the DMG release: generate a signed appcast.xml from all release DMGs (v0.8.3+) with release notes embedded from CHANGELOG.md, then deploy it to GitHub Pages. Deltas disabled (--maximum-deltas 0).
• GitHub release notes now come from the tag's CHANGELOG.md section (fallback: --generate-notes), so one write shows up in both the release page and the update dialog.
• CHANGELOG.md added, built from the existing GitHub release notes (v0.3.0–v0.8.2).

Mac App Store target:

• New ComputerSolitaireAppStore target + shared scheme: macOS-only, same sources, same bundle ID/product name, no Sparkle package, no Sparkle entitlements, own Info-AppStore.plist without Sparkle keys.
• CI gains a mas-build job so the Sparkle-free path compiles on every PR.

Versioning:

• CURRENT_PROJECT_VERSION now mirrors MARKETING_VERSION (0.8.3), single-sourced at the project level so both targets inherit. Enforced by tag-version.sh and the release workflow — Sparkle compares CFBundleVersion, which was previously stuck at 1.

Why

Users currently have no way to learn about or install new versions of the GitHub build. Sparkle gives the direct-download channel in-app updates; the Mac App Store build (which cannot ship Sparkle) gets a dedicated target that compiles the updater away via canImport(Sparkle), with CI keeping that path green.

UI Changes

• New "Check for Updates…" item in the app menu after About (direct build only).
• New Updates section with an "Automatically check for updates" toggle at the bottom of Settings → General (direct build only); General pane height adjusted.
• Sparkle's standard update dialog appears when an update is found.

Validation

• macOS Debug build: Sparkle.framework embedded, signed entitlements carry sandbox + network-client + both mach-lookup exceptions; versions read 0.8.3/0.8.3.
• Mac App Store target build + archive: no Sparkle.framework, no SU* Info.plist keys, sandbox-only entitlements, versions inherited correctly.
• iOS Simulator build: succeeds, no Sparkle embedded, no Frameworks directory.
• Unit tests: pass (xcodebuild test, macOS).
• End-to-end update test: built 0.8.3 with a test EdDSA key, packaged a 0.9.9 DMG, generated a signed appcast, served locally, and confirmed the sandboxed app checked, downloaded, validated, and installed the update in place (0.8.3 → 0.9.9) via the Installer XPC service.
• generate_appcast flags validated against the same pinned Sparkle 2.9.4 tools CI downloads (checksum-verified); changelog section extraction verified for every existing version.
• Workflow YAML and shell syntax validated.

Remaining setup (outside this PR): repo Settings → Pages → Source: "GitHub Actions". Note the first Sparkle-capable release is v0.8.3 — 0.8.2 installs predate Sparkle, so the first real over-the-air update is 0.8.3 → 0.8.4.

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