linter: whitespace, modernize, ineffassign, wastedassign

[mzihlmann]

Bundles four small, mechanical, easy-to-review linter cleanups carved from #539 (kept together since each is trivial):

• whitespace: drop unnecessary leading/trailing blank lines in function bodies
• modernize: strings.Split range loop becomes strings.SplitSeq
• ineffassign / wastedassign: remove assignments overwritten before use (uid, gid := -1, -1, val = "", r := "" become plain var or are removed)

All behaviour-preserving and generated/derived from golangci-lint. The linters themselves are not enabled in the config here, that stays in #539.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/util/assert.go	0.00%	1 Missing ⚠️

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