NIP-13 proof-of-work for geohash channels: mine on send, relax rate limits for PoW senders

Outgoing kind-20000 geohash messages mine a NIP-13 nonce tag (8 leading
zero bits, ~256 hashes, typically <1 ms) off the main actor before
signing. Mining is hard-capped at 2 s and cancellable (newer send or
channel switch): on cap/cancel the committed target steps down so the
message still ships promptly with an honest commitment - sending is
never blocked and nothing is dropped. The hot loop serializes the
canonical event once and rewrites only the fixed-width nonce bytes.

Inbound kind-20000 events are scored per NIP-13 commitment semantics
(committed target counts; the ID must actually meet it, extra work
earns nothing) and never hard-rejected: validated PoW >= 8 bits skips
the per-sender rate-limit bucket while the per-content flood bucket
still applies, so old non-mining clients keep working under today's
strict limits while bulk spam gets expensive.

Presence heartbeats (kind 20001), kind-1 notes, and DMs are unchanged;
no UI beyond a pow= field in an existing sampled debug log.

Reimplemented from scratch rather than cherry-picking the stale
feature/pow-geohash-mining-ui branch (unbounded loop, hard receive
filtering, mining UI, XCTest, force unwraps).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bitchatTests/Performance/PerformanceBaselineTests.swift

@@ -611,6 +611,7 @@ private final class PerfNostrContext: ChatNostrContext {
 
     private(set) var handledPublicMessageCount = 0
     func handlePublicMessage(_ message: BitchatMessage) { handledPublicMessageCount += 1 }
+    func handlePublicMessage(_ message: BitchatMessage, powBits: Int) { handledPublicMessageCount += 1 }
     func checkForMentions(_ message: BitchatMessage) {}
     func sendHapticFeedback(for message: BitchatMessage) {}
     func parseMentions(from content: String) -> [String] {