chore: add unit tests for quality pipeline core functions

[yvonnedevlinrh]

Context

Several core functions in internal/quality/ carry high CRAP scores due to complexity combined with incomplete test coverage. Two functions (WriteText and traceForwardDataFlow, both CC 32) need decomposition before targeted testing. One unexported function (generateSuggestion) has zero direct test coverage.

Related: #166 (Phase 2b extracted traceForwardDataFlow from matchContainerUnwrap at CC 32, higher than the design estimate of ~12)

Functions

Function	File	CC	CRAP	Current Coverage	Approach
WriteText	internal/quality/report.go	32	39.9	80.3% (6 direct tests, but SSA diagnostics + package summary sections untested)	Decompose into section helpers (target CC ≤ 15), then add targeted tests for extracted helpers
traceForwardDataFlow	internal/quality/mapping.go	32	32.4	92.8% (3 direct tests, but transformation call path + multi-iteration convergence untested)	Decompose (extract rhsReferencesAnyTracked, handleTransformationCalls, extractDataFlowLHS), then add targeted tests
generateSuggestion	internal/quality/overspec.go	~6	medium	0% direct (indirect only via ComputeOverSpecification)	Add table-driven tests for all 5 switch cases + default

Removed from scope (already adequately tested):

• ComputeContractCoverage (coverage.go) — 5 direct unit tests since initial implementation (f8d7db0)
• ComputeOverSpecification (overspec.go) — 2 direct unit tests
• mapAssertionsToEffectsImpl (mapping.go) — 2 direct tests via export_test.go + 20+ indirect tests

Acceptance Criteria

• generateSuggestion has a table-driven test covering all 5 switch cases and the default branch
• WriteText complexity reduced to ≤ 15 via helper extraction (e.g., SSA diagnostics section, package summary section, style computation)
• traceForwardDataFlow complexity reduced from 32 to ≤ 12 (extract rhsReferencesAnyTracked, handleTransformationCalls, extractDataFlowLHS)
• Extracted helpers have direct unit tests covering previously untested paths
• All tests run without testing.Short() guards
• go test -race -count=1 -short ./internal/quality/... passes
• golangci-lint run reports zero issues