v1.6.4: fix Full-mode L7 muxer not batching ops (#231)

The batch-build loop blocked on a 30 ms timeout for the first message,
then drained whatever else was in the channel via try_recv() and fired
the batch. Under any non-bursty workload, the channel queue was always
empty by the time the first op woke us up — so every "batch" had
exactly one op, defeating the entire batching premise. Reporter (w0l4i)
saw `batch: 1 ops → ..., rtt=6.3 s` repeating in logs even under high
concurrency.

Fix: after the first op lands, hold the buffer open for an 8 ms
coalescing window. Concurrent ops (parallel fetches, HTTP/2 stream
openings, etc.) now accumulate into the same batch. 8 ms is rounding
error against the 2–7 s Apps Script RTT we're amortizing, and restores
the multi-op-per-batch behavior the rest of the code already supports
(MAX_BATCH_OPS=50, MAX_BATCH_PAYLOAD_BYTES=4 MiB).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/build.gradle.kts

@@ -14,8 +14,8 @@ android {
         applicationId = "com.therealaleph.mhrv"
         minSdk = 24 // Android 7.0 — covers 99%+ of live devices.
         targetSdk = 34
-        versionCode = 142
-        versionName = "1.6.3"
+        versionCode = 143
+        versionName = "1.6.4"
 
         // Ship all four mainstream Android ABIs:
         //   - arm64-v8a      — 95%+ of real-world Android phones since 2019