docs(01-01): amend REQ-video-ring-buffer per D-A4

Replace the REQ-video-ring-buffer bullet to bind the new
getDisplayMedia + offscreen-document acquisition path and drop the
'active-tab' wording (the new API is screen/window-scoped, not
tab-scoped, so there is nothing to re-attach on tab switch). Encoding,
buffer window, and SPEC §10 acceptance citations are unchanged. Adds
CON-display-capture-binding alongside the existing constraint bindings.

.planning/REQUIREMENTS.md

@@ -17,13 +17,19 @@ Requirements for the Phase 1 SPEC. Each maps to one phase in ROADMAP.md.
 ### Video
 
 - [ ] **REQ-video-ring-buffer**: The extension maintains an in-memory ring
-  buffer containing the most recent 30 seconds of active-tab video. Video is
-  captured via `chrome.tabCapture.capture()` at `video/webm; codecs=vp9` @
-  400 000 bps with a `MediaRecorder` timeslice of 2000 ms. A single continuous
-  recorder per session; tab activation changes trigger re-attach. The first
-  emitted chunk (WebM header) is retained indefinitely; subsequent chunks
-  rotate out by the 30-second TTL rule. Bindings: DEC-003, DEC-009,
-  CON-video-window, CON-video-codec, CON-webm-header-retention.
+  buffer containing the most recent 30 seconds of captured video. AMENDED in
+  Phase 01: video is acquired via `navigator.mediaDevices.getDisplayMedia()`
+  invoked from the offscreen document (with `chrome.offscreen.Reason.DISPLAY_MEDIA`),
+  NOT `chrome.tabCapture` as originally specified. The captured stream is
+  screen-or-window-scoped per the operator's one-time selection in Chrome's
+  native picker, and continues unchanged across tab switches. Encoding is
+  unchanged: `video/webm; codecs=vp9` @ 400 000 bps with a `MediaRecorder`
+  timeslice of 2000 ms; a single continuous recorder runs for the whole
+  session. The first emitted chunk (WebM header) is retained indefinitely;
+  subsequent chunks rotate out by the 30-second TTL rule. Bindings: DEC-003
+  (AMENDED), DEC-009, CON-video-window, CON-video-codec,
+  CON-webm-header-retention, CON-display-capture-binding (replaces RETIRED
+  CON-tab-capture-binding).
   - SPEC §10 acceptance criteria: #2, #3, #7.
 
 ### DOM Capture