docs(operations): record the two experimental GET-path switches (#6041)

docs/operations/get-path-experimental-switches.md

@@ -0,0 +1,35 @@
+# GET Path Experimental Performance Switches
+
+This document records two experimental environment switches on the object GET
+path. Both default to **off**, are read once at startup, and exist to support
+staged performance work — they are not general tuning knobs. Until this
+document existed they were referenced only by performance harness scripts,
+which made them look like orphans during dead-code sweeps; they are kept
+deliberately (rustfs/backlog#1832).
+
+## RUSTFS_GET_SEEK_BUFFER_ENABLE
+
+- Type: boolean (`true`/`false`), default `false`.
+- Read once at startup in `rustfs/src/app/object_usecase.rs`.
+- When enabled, small GET responses may be served through an in-memory seek
+  buffer, providing seek support without re-reading the object. The seek-buffer
+  code path is unit-test gated; whether the path stays or graduates to default
+  is a post-1.0 maintainer decision — do not remove either the switch or the
+  gated path as dead code.
+
+## RUSTFS_GET_OUTPUT_HANDOFF_ATTRIBUTION_ENABLE
+
+- Type: boolean (`true`/`false`), default `false`.
+- Read once at startup in `rustfs/src/app/object_usecase.rs`.
+- When enabled, GET responses attribute output-handoff stage timing in the GET
+  stage metrics, at a small per-request bookkeeping cost. Used by the A/B
+  performance runbooks (`scripts/run_get_codec_streaming_smoke.sh`,
+  `scripts/test_get_1mib_abba_stage_metrics.sh`) to compare handoff cost
+  between configurations.
+
+## Operational guidance
+
+Leave both switches unset in production. Enable them only when following a
+performance runbook that asks for them, and unset them afterwards — both are
+startup-latched, so changing a value requires a process restart to take
+effect.