feat: add bounded mixed upload and download queue

[hkimw]

Mixed uploads and downloads can now share a caller-owned queue with bounded chunk scheduling, per-connection admission, pause/resume, rate limits and observable verified completion. Small eligible transfers can progress while large transfers yield, another connection is saturated, or a retry backs off.

The queue reuses the existing upload and artifact verifiers. Only identical chunk operations retry SDK-classified 429/503, network failures and request timeouts within attempt/deadline budgets. Begin, commit and sink operations are never automatically replayed. Progress separates acknowledged, attempted, retried and resumed bytes; 100% remains unverified until a final receipt passes. Upload session limits, partial retention, cancellation and live-instance pause semantics are explicit.

• Adds public types, two schemas, documentation, independent installed examples, and Node/browser mixed-transfer benchmarks.
• Local full check passed (330 tests before the final three queue cases); the final focused artifact/queue suite covers 32 cases. Windows/Linux × Node 22/24 CI runs the complete final suite.
• Real Chrome 154 verified a 2 MiB File upload with pause/resume and lost-ack retry alongside large/small OPFS downloads; all three completed with full verification. A 4 KiB result finished before the large transfer. Evidence is fixture-specific, not a throughput guarantee.
• This remains the 0.10 development checkout. Public archive and official Docs publication are part of the following integration change.