test(terminal): pin cached OSC terminator reuse across BEL frames

Document that forward match reuse requires a monotonic search offset and cover a distant ST held across many intervening BEL frames.

config/reliability-gates.jsonc

@@ -123,6 +123,7 @@
           "file": "src/shared/agent-status-osc.test.ts",
           "assertions": [
             "uses the earliest mixed terminator and counts only parsed payload offsets",
+            "keeps a distant ST usable after many intervening BEL frames",
             "preserves every split of prefixes, JSON, and both terminators across independent streams",
             "applies the pending cap only to incomplete frames"
           ]

src/shared/agent-status-osc.test.ts

@@ -134,6 +134,26 @@ describe('createAgentStatusOscProcessor', () => {
     }
   })
 
+  it('keeps a distant ST usable after many intervening BEL frames', () => {
+    const count = 200
+    const bel = Array.from(
+      { length: count },
+      (_, index) => `\x1b]9999;{"state":"working","prompt":"${index}"}\x07`
+    ).join('')
+    const result = createAgentStatusOscProcessor()(
+      `${bel}\x1b]9999;{"state":"done","prompt":"last"}\x1b\\tail`
+    )
+
+    expect(result.payloads).toEqual([
+      ...Array.from({ length: count }, (_, index) => ({
+        state: 'working',
+        prompt: String(index)
+      })),
+      { state: 'done', prompt: 'last' }
+    ])
+    expect(result.cleanData).toBe('tail')
+  })
+
   it('applies the pending cap only to incomplete frames', () => {
     const marker = '\x1b]9999;{"state":"working"}'
     const atCap = marker + ' '.repeat(64 * 1024 - marker.length)

src/shared/agent-status-osc.ts

@@ -31,6 +31,8 @@ function findAgentStatusTerminator(
   next: { belIndex: number; stIndex: number }
 ): { index: number; length: 1 | 2 } | null {
   // Reuse forward matches, including absence, for this chunk's remaining frames.
+  // Requires `searchFrom` to increase on every call for one `data`; a rewind would
+  // reuse a match that is no longer the earliest.
   if (next.belIndex !== -1 && next.belIndex < searchFrom) {
     next.belIndex = data.indexOf('\x07', searchFrom)
   }