feat(replay): default to skipping recorded delays on replay

BUFFR_REPLAY_NODELAY now defaults to on, so replays emit chunks/frames
back-to-back instead of re-spending the original generation time. Set
BUFFR_REPLAY_NODELAY=0 to restore the recorded streaming cadence when
the timing itself is under test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Robin Bially

README.md

@@ -173,9 +173,9 @@ All flags have environment variable equivalents; flags take precedence.
 
 <br />
 
-buffr records the wall-clock delay before each SSE chunk / WebSocket frame and reproduces that cadence on replay — faithful, but it re-spends the original generation time (often seconds per call) on every run.
+buffr records the wall-clock delay before each SSE chunk / WebSocket frame. By default these delays are dropped on replay so chunks/frames are emitted back-to-back — payloads are identical, only the inter-chunk timing is gone. This keeps replays fast instead of re-spending the original generation time (often seconds per call) on every run.
 
-Set `BUFFR_REPLAY_NODELAY=1` to emit all chunks/frames back-to-back. Payloads are identical; only the inter-chunk timing is dropped. Leave it unset when the streaming cadence itself is under test.
+Set `BUFFR_REPLAY_NODELAY=0` to reproduce the recorded cadence faithfully — do this when the streaming timing itself is under test.
 
 </details>
 

internal/proxy/http.go

@@ -42,11 +42,13 @@ func matchHost(r *http.Request) string {
 	return ""
 }
 
-// ReplayNoDelay, when BUFFR_REPLAY_NODELAY=1, skips the recorded inter-chunk /
-// inter-frame delays on replay. Tests don't need the original streaming cadence,
-// and replaying it (per-chunk time.Sleep) dominates e2e runtime. Exported so it
-// can be toggled in tests; defaults from the env at startup.
-var ReplayNoDelay = os.Getenv("BUFFR_REPLAY_NODELAY") == "1"
+// ReplayNoDelay skips the recorded inter-chunk / inter-frame delays on replay.
+// Tests usually don't need the original streaming cadence, and replaying it
+// (per-chunk time.Sleep) dominates e2e runtime, so this defaults to on. Set
+// BUFFR_REPLAY_NODELAY=0 to restore the recorded cadence (e.g. when the
+// streaming timing itself is under test). Exported so it can be toggled in
+// tests; defaults from the env at startup.
+var ReplayNoDelay = os.Getenv("BUFFR_REPLAY_NODELAY") != "0"
 
 // EgressTransport is the RoundTripper used for upstream HTTP requests in
 // record/auto mode. It defaults to http.DefaultTransport (system-root TLS