fix: IsCompatible returns false for invalid semver inputs

[Copilot]

IsCompatible silently returned true when both inputs were invalid semver strings — semver.Major returns "" for unparseable input, so "" == "" evaluated to true, causing garbage versions to be reported as compatible.

Changes

• pkg/semverutil/semverutil.go — Add semver.IsValid guard before comparing majors; return false immediately if either version is invalid
• pkg/semverutil/semverutil_test.go / spec_test.go — Add invalid-input cases ("", "not-a-version", "vfoo", mixed valid/invalid)

// Before
IsCompatible("not-a-version", "also-not-a-version") // → true ❌

// After
IsCompatible("not-a-version", "also-not-a-version") // → false ✅
IsCompatible("", "")                                 // → false ✅
IsCompatible("v5.0.0", "v5")                        // → true  ✅ (unchanged)

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❌ Design Decision Gate 🏗️ failed during design decision gate check.

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❌ Test Quality Sentinel failed during test quality analysis.

[Copilot]

Pull request overview

Fixes a correctness bug in pkg/semverutil where IsCompatible could return true for two invalid semver strings due to semver.Major returning "" for unparseable inputs.

Changes:

• Add a validity guard in IsCompatible to return false when either version is not valid semver.
• Add test coverage for invalid/empty version inputs in both unit tests and spec tests.

Show a summary per file

File	Description
pkg/semverutil/semverutil.go	Adds semver.IsValid guard to prevent invalid inputs from being treated as compatible.
pkg/semverutil/semverutil_test.go	Adds unit tests covering invalid/empty semver inputs for IsCompatible.
pkg/semverutil/spec_test.go	Extends public API spec tests with invalid/empty input compatibility cases.

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• Files reviewed: 3/3 changed files
• Comments generated: 1