fix: buffer operations in Stale/StaleWithBuffer instead of throwing (#6342)

When 5+ users edit concurrently, revision gaps trigger the Stale state
which throws 'Ignored server-side change.' on incoming operations. This
causes document divergence, silent data corruption, and eventually
force-resets that discard local edits.

Changes:
- Stale/StaleWithBuffer.applyServer: buffer incoming operations instead
  of throwing, apply them sequentially after missing ops arrive
- Stale.applyClient: carry pendingOps to StaleWithBuffer on transition
- editor-socketio-server: retry-based force-reset (3 failures) instead
  of immediate nuclear reset on any OT error
- hex2rgb: defensive null checks for old non-hex color records

Signed-off-by: bxff <51504045+bxff@users.noreply.github.com>

public/vendor/ot/hex2rgb.js

@@ -1,12 +1,17 @@
 function hex2rgb (hex) {
+  if (!hex || typeof hex !== 'string') return { red: 0, green: 0, blue: 0 }
   if (hex[0] == '#') hex = hex.substr(1)
   if (hex.length == 3) {
     var temp = hex
     hex = ''
-    temp = /^([a-f0-9])([a-f0-9])([a-f0-9])$/i.exec(temp).slice(1)
+    var match = /^([a-f0-9])([a-f0-9])([a-f0-9])$/i.exec(temp)
+    if (!match) return { red: 0, green: 0, blue: 0 }
+    temp = match.slice(1)
     for (var i = 0; i < 3; i++) hex += temp[i] + temp[i]
   }
-  var triplets = /^([a-f0-9]{2})([a-f0-9]{2})([a-f0-9]{2})$/i.exec(hex).slice(1)
+  var result = /^([a-f0-9]{2})([a-f0-9]{2})([a-f0-9]{2})$/i.exec(hex)
+  if (!result) return { red: 0, green: 0, blue: 0 }
+  var triplets = result.slice(1)
   return {
     red: parseInt(triplets[0], 16),
     green: parseInt(triplets[1], 16),