fix: notify on new Xcode version identity, not array-count growth

[YuriNachos]

Problem

The "New Xcode version available" notification fired whenever the availableXcodes array count grew (newValue.count > availableXcodes.count), not when a genuinely new version identity appeared. This caused two user-visible bugs:

• False negative — when a new version appears in the same refresh that an old one is removed (e.g. a data source drops an obsolete beta row the moment it adds the new release), the count is unchanged, so the user is never told a new Xcode landed (the core reason to run the app).
• False positive — a data-source switch or a duplicate row grows the count without a new version identity, firing a spurious banner.

Solution

Extract a pure helper that compares version identity, not array length:

static func newlyAvailableXcodes(oldXcodes: [AvailableXcode], newXcodes: [AvailableXcode]) -> [AvailableXcode] {
    guard !oldXcodes.isEmpty else { return [] }
    let oldIDs = Set(oldXcodes.map(\.xcodeID))
    return newXcodes.filter { !oldIDs.contains($0.xcodeID) }
}

availableXcodes.willSet now notifies only when the helper's result is non-empty:

if !Self.newlyAvailableXcodes(oldXcodes: availableXcodes, newXcodes: newValue).isEmpty { ... }

• AvailableXcode.xcodeID (already Hashable) is the identity, so the same version with a different architecture correctly stays distinct and does NOT re-notify.
• The initial load (empty → populated) is suppressed: oldXcodes.isEmpty → [], so first-run does not banner every Xcode.
• No Localizable.xcstrings change (title/body keys unchanged).

Testing

New XcodesTests/NewVersionNotificationTests.swift (own file to avoid the AppStateTests.swift conflict with PR #840) asserts directly on the pure helper:

• initial-load suppression (empty → populated = []);
• true positive (one genuinely new version);
• false negative (new added + old removed, count unchanged → the new version IS reported);
• false-negative variant (new appears while list shrinks);
• false positive ([A] → [A, A] → [], no spurious notify);
• identical list (no notify);
• architecture-distinct same version (no re-notify).

Real red-before-green: on main's count predicate, the equal-count cases (new+removed) notify nothing, so the test expecting the new version's identity fails; after the fix it passes.

xcodebuild test -scheme Xcodes -destination 'platform=macOS' CODE_SIGN_IDENTITY="" CODE_SIGNING_REQUIRED=NO CODE_SIGNING_ALLOWED=NO -only-testing:XcodesTests/NewVersionNotificationTests → TEST SUCCEEDED.

Checklist

• Diff scoped to AppState.swift (predicate) + new NewVersionNotificationTests.swift + project.pbxproj (test-target registration only).
• No overlap with PR fix: duplicated major version in install notification title #840 (AppState+Install.swift) or fix: rebind Create Symbolic Link shortcut to Cmd+Opt+L to resolve Make Active collision #841 (XcodeCommands.swift).
• Localizable.xcstrings byte-identical to main.
• No new compiler warnings.

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Authored by @YuriNachos. Implementation written by a cccc (Claude Code, GLM-5.2) worker under orchestrator acceptance; an independent adversarial review returned APPROVE with no blocking findings.