Test downloader canonical feed handling

[ktsaou]

Summary

• add behavior tests for downloader canonical parse, render, and prepare paths
• cover sanitizer handling for BOM, CRLF, blank lines, 0.0.0.0, and /0 inputs
• raise pkg/downloader coverage by covering canonical.go without production changes

Validation

• go test ./pkg/downloader
• go test -coverprofile=/tmp/update-ipsets-downloader-slice27.cover -covermode=atomic ./pkg/downloader
• go run ./tools/archposture -root . > /tmp/update-ipsets-archposture-slice27.json
• make lint
• make staticcheck
• make golangci-lint
• CI=true make coverage
• make test-strict
• git diff --check

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 40 complexity · 2 duplication

Metric	Results
Complexity	40
Duplication	2

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AI Reviewer: first review requested successfully. AI can make mistakes. Always validate suggestions.

_{TIP This summary will be updated as you push new changes.}