Bound Codex proxy mirroring and debug log pressure during Responses streams #372

Problem

The heap-crash investigation found a separate pressure amplifier from #369: packages/codex-headless/src/proxy/responsesProxy.ts mirrors every proxy event synchronously, and response-chunk events serialize raw Buffers through JSON/base64 before normal listeners run. This creates large transient strings, blocks the event loop, and writes unbounded active proxy-events.jsonl files.

Local evidence from the investigation found proxy storage near 965 MiB, with individual proxy-events.jsonl files over 100-280 MiB. Feed-debug and performance logging have similar per-file cap gaps.

Findings

• ResponsesProxy.emit mirrors before super.emit, so slow/string-heavy mirror writes delay the stream adapter.
• response-chunk mirroring can hold raw bytes, base64 text, JSON text, and append buffers at once.
• Request capture buffers and parses full bodies for shape extraction and may base64 encode request bodies.
• Retention eventually prunes disk, but active writers have no per-run byte cap or drop mode.
• Feed-debug renderer memory is count-capped, but disk files are not per-file byte-capped.
• Retention sweeps are cleanup, not pressure control, and can themselves be expensive during pressure.

Proposed Fix

• Stop raw/base64 mirroring of every response-chunk by default. Store metadata plus bounded samples/tails; gate full raw capture behind an explicit forensic flag.
• Add per-run/per-file byte caps and dropped counters for proxy mirrors, feed-debug, and performance logs.
• Move mirror writes off the hot synchronous path into a bounded async queue/drop policy.
• Add lightweight counters: chunk bytes, serialized bytes, mirror write ms, cap drops, queue depth, per-run bytes.
• Make retention cheaper and visible: one scan per prune, protected bytes, active-grace skips, removals, and scan duration.

Acceptance

• Large Responses streams no longer synchronously base64/JSON mirror every chunk by default.
• Active proxy/feed/perf log files have hard caps and record dropped counters.
• A stream stress test demonstrates bounded proxy mirror memory/disk behavior.
• Debug/retention metrics make it clear when diagnostics are being dropped because of caps.

Related: #327, #365, #369, #370