fix(core): disable QUIC segmentation offload on Android

[momics]

This was generated by AI during implementation.

Why

Android can advertise UDP segmentation offload and then reject a batched sendmsg with an I/O error. The currently released noq disables GSO after that rejection but does not retransmit the rejected application packets, which can stall request bodies.

What changed

• disable QUIC segmentation offload only when the endpoint is compiled for Android
• preserve the upstream default on iOS, macOS, Linux, and Windows
• consolidate the duplicated transport-builder branches while retaining idle-timeout behavior
• add focused platform-policy tests
• link the temporary policy directly to [noq-udp] Retransmit failing GSO packets and split batches n0-computer/noq#746

This is intentionally a narrow compatibility policy, not a custom UDP implementation. Issue #388 tracks its removal after the fixed noq release is adopted and revalidated.

Verification

• cargo test -p iroh-http-core --lib: 97 passed
• focused transport-policy tests: 2 passed
• strict library clippy: passed
• Android debug APK with embedded frontend: built and installed
• three consecutive physical-device Android-to-desktop suites: 107 pass / 0 fail / 0 skip each
• body latency ranges: 4 KiB 29.9-32.2 ms; 64 KiB 39.8-82.5 ms; 1 MiB 227.3-251.1 ms; 5 MiB 796-919.3 ms
• no QUIC application-data send errors occurred

One independent 60-byte QADv4 connectivity probe still observed Android's kernel rejection and immediately disabled its own GSO path. That probe is outside the QUIC transport configuration, did not affect application data, and remains upstream-owned.

Closes #387.
Follow-up: #388.
Upstream: n0-computer/noq#746.