chore: add dependency lower-bound and cognitive complexity CI workflows

[kevmoo]

Summary

• Adds .github/workflows/lower_bound.yml (dependency floor validation).
• Adds .github/workflows/complexity.yml (cognitive complexity scanner).

[github-actions]

📦 Dependency Lower-Bound Validation Summary

✅ source_gen_test

• SDK Floor: 3.9.0
• Status: Clean

Dependency	Declared Constraint	Declared Floor	Resolved Version	Status
analyzer	>=9.0.0 <15.0.0	9.0.0	9.0.0	Exact Floor
build	^4.0.3	4.0.3	4.0.3	Exact Floor
build_test	^3.5.4	3.5.4	3.5.4	Exact Floor
dart_style	^3.1.3	3.1.3	3.1.3	Exact Floor
meta	^1.16.0	1.16.0	1.16.0	Exact Floor
path	^1.9.0	1.9.0	1.9.0	Exact Floor
source_gen	^4.1.1	4.1.1	4.1.1	Exact Floor
test	^1.27.0	1.27.0	1.27.0	Exact Floor

[github-actions]

📊 Cognitive Complexity Analysis

Net Delta: 0 | Added: 0 | Increased: 0 | Improved: 0 | Violations: 0

No modified Dart declarations detected.