Fix all golangci-lint issues with extended linters

[andrew]

Resolves all 15 warnings from golangci-lint when run with gocritic, gocognit, gocyclo, maintidx, dupl, mnd, unparam, ireturn, goconst, and errcheck enabled.

• Replace wrapper lambdas with direct function references (gocritic unlambda)
• Use += operator instead of x = x + y (gocritic assignOp)
• Extract operator string constants to fix goconst
• Refactor normalizeTokens into a tokenNormalizer struct to reduce cognitive complexity from 62 to under 30 (gocognit) and convert if-else chain to switch (gocritic ifElseChain)
• Replace magic number with named variable (mnd)
• Allow Expression interface returns in golangci config (ireturn)
• Add nolint directive for large table-driven test (maintidx)