Parallel SSE-only network quality (draft: 20s end-to-end gate not met)

[lop-spec]

Changes

• Default catalog-wide parallel SSE-only testing; no download or model requests.
• Preserve the full 401-frame, 50ms, 20s profile and validation thresholds.
• Keep explicit legacy bandwidth mode serial; retain successful history and archived download results.
• Preserve cancellation, isolated-core cleanup, and source/local-measurement failure attribution.

Validation and release gate

• Local loopback fixture: all 179 streams valid, peak concurrency 179, no download requests, 20.55s total. This is not a real subscription-route benchmark.
• The explicit <=20000ms end-to-end assertion FAILED. No shortened samples or relaxed threshold.
• Smart Proxy local tests: 91/91 passed.
• Real-route parallel accuracy still needs verification.
• Draft only: do not merge or release. Production Worker/Pages and the daily EXE/core are unchanged by this candidate.