test(e2e): instrument mock SSE + client stream + fix reachability probe to attribute #90 mode-B (#98)

Adds diagnostics-only instrumentation to attribute the Activation E2E
positive flow's "SSE reply never renders" failure (issue #90 mode B)
between (a) expo/fetch not streaming the SSE response on the Android
release APK, vs (b) a mock-side broadcast bug. Does not change app
behavior or fix the root cause — #90 stays open pending the next CI
run's enriched logs.

- tests/fixtures/mock-opencode-server.ts: per-request logging
  (method/path/status), per-SSE-connection connect/disconnect logging
  with live client count, per-broadcast event-type + client-count
  logging, and a 2s SSE heartbeat comment so client-side silence
  becomes unambiguous.
- src/lib/sdk.ts global.events(): logs on the first successful
  reader.read() that returns data, and when the stream loop ends —
  proves/disproves whether expo/fetch ever delivers a byte.
- scripts/run-e2e-flows.sh: the emulator->mock reachability probe used
  toybox wget/nc, which don't work reliably on the API-28 image.
  Replaced with a probe chain (curl, wget, mksh /dev/tcp, nc, then a
  host-side fallback) that writes a clear PASS/FAIL/UNKNOWN verdict to
  artifacts/diag/probe.txt without blocking the flow.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/lib/sdk.ts

@@ -253,10 +253,19 @@ export function createClient(config: ClientConfig) {
         const decoder = new TextDecoder()
         const parser = new SSEParser()
 
+        let receivedFirstByte = false
         try {
           while (true) {
             const { done, value } = await reader.read()
-            if (done) break
+            if (done) {
+              console.log("[SSE] stream ended")
+              break
+            }
+
+            if (!receivedFirstByte) {
+              receivedFirstByte = true
+              console.log(`[SSE] first byte received (${value?.byteLength ?? 0} bytes)`)
+            }
 
             for (const data of parser.push(decoder.decode(value, { stream: true }))) {
               try {