Fix BLE stream crashes and gossip sync races (#663)

* Handle long BLE packets safely

* Keep BLE stream aligned after partial drops

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchatTests/GCSFilterTests.swift

@@ -0,0 +1,22 @@
+import XCTest
+@testable import bitchat
+
+final class GCSFilterTests: XCTestCase {
+    func testBuildFilterWithDuplicateIdsProducesStableEncoding() {
+        let id = Data(repeating: 0xAB, count: 16)
+        let ids = Array(repeating: id, count: 64)
+
+        let params = GCSFilter.buildFilter(ids: ids, maxBytes: 128, targetFpr: 0.01)
+        XCTAssertGreaterThanOrEqual(params.m, 1)
+
+        let decoded = GCSFilter.decodeToSortedSet(p: params.p, m: params.m, data: params.data)
+        XCTAssertLessThanOrEqual(decoded.count, 1)
+    }
+
+    func testBucketAvoidsZeroCandidate() {
+        let id = Data(repeating: 0x01, count: 16)
+        let bucket = GCSFilter.bucket(for: id, modulus: 2)
+        XCTAssertNotEqual(bucket, 0)
+        XCTAssertLessThan(bucket, 2)
+    }
+}