feat(apple): adapt the macOS client to ABI v2 — client identity + SPAKE2 PIN pairing

The pairing/renegotiation batch bumped the punktfunk/1 ABI to v2 and the host now
hard-rejects v1 Hellos (m3.rs), so streaming from the Mac was dead until the bundled
PunktfunkCore.xcframework is rebuilt — it is gitignored, so that is a per-checkout step:
bash scripts/build-xcframework.sh. The Swift wrapper itself was already adapted upstream;
this lands the app on top of it.

- ClientIdentityStore: persistent client identity in the login Keychain, presented on
  every connect so paired hosts recognize this Mac. Keychain access failure throws
  instead of regenerating (a fresh identity would silently un-pair this Mac from every
  --require-pairing host); a lost first-run race resolves toward the stored identity;
  pairing uses the strict loadForPairing() so a memory-only identity can't strand a
  ceremony.
- PairSheet: the SPAKE2 PIN ceremony, reachable from a host card's context menu and from
  the trust prompt's "Pair with PIN instead…" (which drops the live session first — the
  host's accept loop is sequential). Success pins the verified fingerprint and connects;
  an in-flight ceremony self-discards when the sheet is dismissed, so a late success
  can't pin + auto-connect behind the user's back. Wrong PIN and Keychain failures get
  distinct, actionable error text.
- Tests: identity unit tests; the full pairing ceremony + --require-pairing gate on
  loopback (test-loopback.sh arms a second host, parses its PIN from the log, and gives
  both hosts throwaway config homes — no more writes to the real ~/.config/punktfunk);
  remote pairing + pinned stream over the LAN (PUNKTFUNK_REMOTE_PIN, _PORT).

Validated live against the box: SPAKE2 ceremony with the host's arming PIN → verified
fingerprint → pinned + identified 720p60 session (host persisted the client identity);
first light 60/60 AUs decoded to pixels; vkcube on glass through the app.

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

clients/apple/Tests/PunktfunkKitTests/IdentityTests.swift

@@ -0,0 +1,36 @@
+// Client identity generation through the ABI (punktfunk_generate_identity): the PEM pair
+// hosts use to recognize a paired client. Pure local crypto — no host needed.
+
+import XCTest
+@testable import PunktfunkKit
+
+final class IdentityTests: XCTestCase {
+    func testGenerateIdentityYieldsDistinctPEMPairs() throws {
+        let a = try generateIdentity()
+        let b = try generateIdentity()
+
+        XCTAssertTrue(a.certPEM.contains("BEGIN CERTIFICATE"), "cert is PEM")
+        XCTAssertTrue(a.keyPEM.contains("PRIVATE KEY"), "key is PEM")
+        XCTAssertTrue(a.certPEM.hasSuffix("\n") || a.certPEM.contains("END CERTIFICATE"))
+
+        // Each call mints a fresh keypair — identical output would mean a broken RNG.
+        XCTAssertNotEqual(a.certPEM, b.certPEM)
+        XCTAssertNotEqual(a.keyPEM, b.keyPEM)
+    }
+
+    func testPairAgainstNothingFailsCleanly() {
+        // Nothing listens on this port; the ceremony must throw within its timeout, and
+        // must not report .wrongPIN (no SPAKE2 exchange ever happened).
+        do {
+            let identity = try generateIdentity()
+            _ = try pair(
+                host: "127.0.0.1", port: 9, identity: identity,
+                pin: "0000", name: "test", timeoutMs: 2000)
+            XCTFail("expected pair() against a dead port to throw")
+        } catch PunktfunkClientError.wrongPIN {
+            XCTFail("dead port must not look like a wrong PIN")
+        } catch {
+            // any other error is the correct outcome
+        }
+    }
+}