fix: correct PDF y-axis transform

[besmpl]

Summary

• compose the PDF page transform in scale-then-translate order
• apply the same top-left-to-bottom-left mapping in Backend.Begin and Document.NewPage
• add regressions that assert the exact composed transform implementing (0, 0) → (0, height) and (0, height) → (0, 0)

Why

Transform.Then applies the receiver before the next transform. Translating before the negative Y scale produced y' = -height - y, placing gg coordinates below the page. Scaling first and translating second produces the intended y' = height - y.

Verification

• go test ./... -count=1
• go test -race ./... -count=1
• go build ./...
• go vet ./...
• gofmt and git diff --check
• local added executable-line patch coverage: 100% (2/2)

Upstream GitHub Actions and codecov/patch are configured but await maintainer approval of the fork workflow run.

[kolkov]

Correct fix. Transform.Then applies the receiver first, so Translate(0, h).Then(Scale(1, -1)) was producing y' = -(y + h) instead of the intended y' = h - y. Scale(1, -1).Then(Translate(0, h)) gives the right top-left → bottom-left mapping.

Both Backend.Begin and Document.NewPage fixed consistently. Tests assert the exact composed transform — good regression coverage.

LGTM, merging.

[codecov]

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