fix(viewer): actively cancel superseded PDF.js work on current parent

[seonghobae]

Objective

Reconstruct stale stacked PR #347's unique active PDF.js resource-cancellation delta on the exact current #323 parent without moving #323 or importing obsolete ancestry.

Exact stack identity

• parent PR: fix(viewer): suppress stale PDF render publication #323;
• exact parent head/base: 75e49391ddc13de378b8572d444bf02d686ef84c;
• test-only RED commit: 4fd6398fdba14cc72a0103cdcac0bdcd5a9b3d6e;
• exact GREEN candidate head: 54d77fbc07645a8eee45ad8662700c3c0845ccc9;
• changed paths: scripts/test_viewer_pdfjs_resource_cancellation.py and viewer.js;
• emitted child CI and fuzz: terminal-success;
• unresolved review threads: zero.

RED → GREEN contract

The executable Node/unittest regression holds an active RenderTask.promise and an unresolved PDFDocumentLoadingTask.promise. Supersession must call RenderTask.cancel() exactly once and loadingTask.destroy() exactly once when those APIs exist. Production installs a one-shot abort listener, cancels active render work, destroys unresolved loading work, suppresses cancellation errors only when the caller actually aborted, removes the listener in finally, and still destroys a resolved PDFDocumentProxy.

Scope boundary

This child depends on #323's stale-publication suppression. It does not yet provide signed-token lifecycle parity, rapid three-generation/error-state coverage, terminal focus ownership, or a distributed/browser-process cancellation contract. Issue #322 remains open.

Merge gate

Keep Draft while #323 remains unmerged. Child CI/fuzz success is useful review evidence but not protected-main merge evidence. After #323 reaches protected main, rebuild or retarget this two-file delta onto the new exact protected base and regenerate all main-targeted CI, Security Scan, SAST, fuzz, current-head reviews, and independent approval. No evidence from #347 or predecessor heads transfers.

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[seonghobae]

Read-only child-head review request for 54d77fbc07645a8eee45ad8662700c3c0845ccc9 over exact parent #323 head 75e49391ddc13de378b8572d444bf02d686ef84c. Emitted CI and fuzz are terminal-success; this remains Draft and is not eligible for protected-main merge evidence. Review deterministic active RenderTask cancellation, unresolved loading-task destruction, listener cleanup, abort-only error suppression, and resolved-document destruction. Do not mutate or merge the branch.

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