fix: honor requested PDF page dimensions

[besmpl]

Summary

• create PDF pages with the exact dimensions passed to Backend.Begin
• preserve per-page dimensions in Document.NewPage
• upgrade gxpdf from v0.2.0 to the minimal v0.4.0 release that exposes NewPageWithDimensions
• stage page creation before mutating backend state so a failed Begin leaves the prior page coherent
• add serialized MediaBox regressions for single-page and mixed-size documents

Why

The backend accepted width and height but always created an A4 page. For example, Begin(320, 240) serialized /MediaBox [0 0 595 842] instead of 320 x 240. Recording playback passes its canvas dimensions directly to Backend.Begin, so the output page size must preserve them.

gxpdf v0.2.0 has no public arbitrary-page-size API. v0.4.0 is the first release with Creator.NewPageWithDimensions, making it the smallest dependency update that can implement the contract cleanly.

Verification

• baseline Begin(320, 240) produces A4 MediaBox
• single-page output now contains the exact requested MediaBox
• mixed-size Document pages retain their independent dimensions
• non-positive dimensions return an error
• a failed second Begin preserves the previously initialized page
• go test -count=1 ./...
• go test -race -count=1 ./...
• go test -count=1 -coverprofile=coverage.out ./...
• local added executable-line patch coverage: 100% (21/21)
• go build ./...
• go vet ./...
• go mod tidy -diff
• gofmt and git diff --check

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

Coordination

This is independent of #2, although both patches touch page initialization and may need a trivial rebase depending on merge order.

Review follow-up (2026-08-12)

Rebased onto the merged lifecycle/y-flip work. The combined implementation preserves Scale(1, -1).Then(Translate(0, height)), retains the injectable page-creation failure seam, and creates every page with its exact requested dimensions.

[kolkov]

Real bug — the backend accepted width/height but always created A4 pages. NewPageWithDimensions is the right fix.

Good defensive detail: staging new state before mutation so a failed Begin leaves the prior page coherent. Non-positive dimension rejection is delegated to gxpdf's own validation rather than duplicated — reasonable since NewPageWithDimensions already guards this. The MediaBox serialization tests verify the actual PDF output, not just internal state.

gxpdf v0.4.0 bump is required for NewPageWithDimensions — confirmed available.

LGTM, merging.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[kolkov]

This PR has a merge conflict after #1, #2, and #4 were merged. The conflict is in Backend.Begin() (the Y-flip transform formula changed in #2) and Document.NewPage().

Could you rebase on the updated main? The conflicts should be straightforward — the flip transform is now Scale(1, -1).Then(Translate(0, h)) per #2.