fix(sync): reject out-of-range GCS parameters instead of decoding garbage

decodeToSortedSet takes p and m straight off the wire - REQUEST_SYNC
carries P as a uint8, so 0..255 all arrive - and decodes with them
regardless. Kotlin's shift operators use the low 6 bits of the count, so
p >= 64 wraps and the decoder emits values that were never in the filter;
p = 0 turns every remainder into a zero-width read and manufactures a
value per bit. On today's 20-id filter, p = 0 decodes 105 values.

Those values are read as "the peer already has this packet", so the
responder withholds packets the peer actually lacks - the failure
direction that loses messages.

Guard the parameters as iOS does in GCSFilter.decodeToSortedSet: p in
1..32 and m > 1, otherwise decode to nothing, which callers read as "peer
has nothing" and answer by sending the data.

app/src/main/java/com/bitchat/android/sync/GCSFilter.kt

@@ -18,6 +18,14 @@ import kotlin.math.ln
  * - Bitstream is packed MSB-first in each byte.
  */
 object GCSFilter {
+    /**
+     * Highest Golomb-Rice parameter accepted from the wire. P maps to an FPR of
+     * ~1/2^P; beyond 32 the remainder width exceeds any practical filter, and
+     * the shifts in decode silently wrap (Kotlin shifts use the low 6 bits of
+     * the count) into garbage values.
+     */
+    const val MAX_P = 32
+
     data class Params(
         val p: Int,         // Golomb-Rice parameter (>= 1)
         val m: Long,        // Range M = N * 2^P
@@ -70,6 +78,11 @@ object GCSFilter {
     }
 
     fun decodeToSortedSet(p: Int, m: Long, data: ByteArray): LongArray {
+        // p and m arrive off the wire. Reject out-of-range parameters rather
+        // than decoding garbage: callers read the result as "peer has nothing"
+        // and fall back to sending the data, which is the safe direction.
+        // Matches the iOS guard in GCSFilter.decodeToSortedSet.
+        if (p < 1 || p > MAX_P || m <= 1L) return LongArray(0)
         val values = ArrayList<Long>()
         val reader = BitReader(data)
         var acc = 0L