perf(p2p-14): add adaptive read-path bypasses (#671)

[jlon]

Summary

• Add adaptive read-side optimizations after the data-plane fast path is isolated.
• Keep scope on selection heuristics and leave telemetry, rollout material, and harness expansion out.

Design

• Apply bypasses only when cached-hit and adaptive signals justify them.
• Keep sample attribution and hygiene explicit so adaptive decisions remain reviewable.

Implementation

• Add cached-hit read-flight bypass and adaptive worker bypass.
• Tighten adaptive sample hygiene and wire the minimal service-side attribution needed for the decision path.

Verification

• Run the production gate with adaptive behavior enabled.
• Check 0ms benchmark sanity and targeted adaptive-path tests.

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