refactor: decompose high-CRAP functions in internal/analysis

[yvonnedevlinrh]

Context

Follow-up to #200 (decompose high-complexity CLI and analysis functions). After that work, gaze identifies three remaining high-CRAP functions in internal/analysis/ with [decompose] fix strategy:

#	CRAP	Function	File	Fix Strategy
1	24.2	detectASTPointerArgMutations	internal/analysis/mutation.go:549	decompose
2	21.0	funcSignature	internal/analysis/returns.go:200	decompose
3	17.3	findSSAFunction	internal/analysis/mutation.go:92	decompose

Acceptance Criteria

• CC ≤ 15 for all three functions
• Extracted helpers have unit tests covering primary branches
• All existing tests pass without modification
• golangci-lint run clean
• CRAPload does not increase

Prior Art

• refactor: decompose high-complexity CLI and analysis functions #200 — same decomposition pattern, successfully reduced 5 functions
• chore: reduce CRAPload fragility — cover 19 zero-coverage functions at CRAP >= 30 #166 — parent CRAPload reduction effort