[Velli20]

Stacked on #593 (both touch the canvas XObject/graphics-state handlers and variants.rs). Retarget to main once #593 merges.

Why

pdf_operator_backend.rs defined twelve per-category operator traits plus the PdfOperatorBackend supertrait. In practice:

• PdfCanvas was the only production implementor; the other was a test-only RecordingBackend.
• No code was generic over any single category trait, and none had default methods, so splitting them didn't let anyone implement just part of the set.
• RecordingBackend turned each parsed PdfOperatorVariant into RecordedOperation, a near-copy of the same enum.

PdfOperatorVariant is already a closed, data-only description of every operator, so the trait layer only added indirection.

What

• Deleted pdf_operator_backend.rs, recording_pdf_operator_backend.rs, PdfOperatorVariant::call and BeginMarkedContentWithProps::call. BeginMarkedContentWithProps now exposes tag() and properties() instead.
• Added PdfCanvas::apply_operator in canvas_operator_dispatch.rs. The canvas_*_ops.rs and shading.rs trait impls are now pub(crate) inherent methods.
• Removed thin wrappers:
  • q/Q call save/restore directly.
  • Rendering intent, flatness, BX/EX and d0/d1 share one no-op arm.
  • The never-dispatched mark_point* handlers are gone.
• Tests check the parsed operators directly instead of replaying them through the recorder. Tests that only exercised dispatch were removed or narrowed to parsing.
• Updated the architecture note in AGENTS.md.

Verification

• cargo fmt --check, cargo clippy --all --workspace and cargo check --no-default-features are clean.
• cargo test --workspace passes, excluding pdf-graphics-skia and pdf-conformance, which need a Skia download that wasn't available locally.
• cargo check --target wasm32-unknown-unknown -p web-canvas-example passes.
• text_recording bench, shipped profile, 5 runs alternating with the previous commit: −0.5% to +0.9% per case, within noise.

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