Document iOS TDLib linking blocker

docs/ios/tdlib-linking.md

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+# TDLib iOS Linking Status
+
+Local toolchain status:
+
+- Xcode 26.5 is installed at `/Applications/Xcode.app`.
+- iOS 26.5 simulator runtime is installed.
+- `scripts/check-ios-prereqs.sh` passes.
+- `scripts/run-ios-simulator-app.sh` launches the fake-backed SwiftUI shell in iOS Simulator.
+
+Current real TDLib blocker:
+
+```bash
+DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer cargo build -p tele-ios-ffi --target aarch64-apple-ios-sim --release
+```
+
+Observed failure:
+
+```text
+[Your OS or architecture may be unsupported.] Failed to download file: Please try using the `pkg-config` or `local-tdlib` features.
+404 Not Found
+https://github.com/FedericoBruzzone/tdlib-rs/releases/download/v1.2.0/tdlib-1.8.29-ios-aarch64.zip
+```
+
+Interpretation:
+
+- Rust, Swift, Xcode, and the iOS simulator runtime are working.
+- The fake-backed iOS app shell can be built, installed, launched, and rendered.
+- Real TDLib iOS linking is blocked because `tdlib-rs` 1.2.0 does not publish the iOS static library artifact requested by its `download-tdlib` build script.
+
+Next viable paths:
+
+1. Build TDLib for `iphoneos` and `iphonesimulator` locally and switch the Rust dependency path to `local-tdlib` or `pkg-config`.
+2. Add a fake-only `tele-ios-ffi` build feature that avoids linking TDLib for simulator UI work, while keeping real TDLib behind a separate feature.
+3. Replace the `tdlib-rs` packaging path for iOS with a lower-level C ABI/XCFramework if UniFFI plus `tdlib-rs` cannot link cleanly on device.