feat: add language-agnostic analysis packages for pyscn parity

[DaisukeYoda]

Summary

• Add 7 new packages (domain/, dfa/, semantic/, cbo/, lcom/, nesting/, source/) and extend clone/ with 3 new files to extract all language-independent analysis logic from pyscn into codescan-core
• This enables pyscn to replace its internal copies with direct imports from codescan-core, reducing duplication and centralizing the shared algorithms
• All new code follows the existing adapter pattern (interfaces + any typed fields) so language-specific logic remains in pyscn/jscan

New packages

Package	Description
domain/	Shared error types (DomainError), RiskLevel, CloneType, OutputFormat, SortCriteria, and all default threshold constants
dfa/	Data flow analysis: DefUseKind, VarReference, DefUsePair, DefUseChain, DFAInfo, DFAFeatures, and DFABuilder with RefExtractor interface for language adapters
semantic/	CFG+DFA semantic similarity: ExtractCFGFeatures, CompareCFGFeatures, CompareDFAFeatures, ComputeSimilarity with configurable weights
cbo/	Coupling Between Objects: ComputeCBO with dependency kind breakdown and risk assessment
lcom/	Lack of Cohesion of Methods (LCOM4): Union-Find algorithm with connected component grouping
nesting/	Nesting depth analysis via NestingClassifier interface for language-agnostic tree traversal
source/	File collection with glob-based include/exclude filtering and recursive directory walking

Extended clone/ package

File	Description
fragment.go	CodeFragment (implements GroupableItem), ClonePair, CloneGroup, CloneStatistics
similarity.go	SimilarityAnalyzer interface + StructuralAnalyzer wrapping APTED
classifier.go	Cascade clone classifier (Type-1 → Type-4) with configurable thresholds

Key design decisions

• dfa.RefExtractor and nesting.NestingClassifier are interfaces so language-specific AST logic stays in pyscn/jscan
• semantic.ComputeSimilarity returns 0.0 for nil CFGs to prevent false-positive matches on parse failures
• dfa.DFABuilder.Build validates the extractor is non-nil before use, returning an error instead of panicking
• No existing files were modified — only new files added

Test plan

• go test ./... -count=1 — all 13 packages pass (24 new files, 4261 lines added)
• go vet ./... — clean
• go build ./... — clean
• Nil-input edge cases covered (nil CFG, nil extractor, nil classifier, empty inputs)

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