NFC protocol issues (CTAP2 keepalive, SELECT Le, status word preservation)

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### Missing CTAP2 keepalive (`SW=0x91xx → NFCCTAP_GETRESPONSE`)
- **Spec ref:** CTAP 2.2 §11.3.4 (NFCCTAP_GETRESPONSE keepalive flow).
- **Location:** `src/transport/nfc/channel.rs:255-304` (`cbor_send`). Contains a literal `// FIXME check for SW_UPDATE?` and a commented-out keepalive block.
- **Behavior:** After the final NFCCTAP_MSG fragment the response goes straight into `CborResponse::try_from`. There is no loop on `(sw1 == 0x91)` that issues NFCCTAP_GETRESPONSE (`CLA=0x80, INS=0x11`). The library's existing 0x61xx loop in `handle()` is the ISO 7816 GET RESPONSE mechanism for response continuation, not the CTAP2 keepalive.
- **Impact:** Any CTAP2 NFC operation that takes long enough for the authenticator to emit a keepalive (on-token UV, large credential lists, biometric capture) is misinterpreted.

### SELECT issued as Case 3 APDU (no Le)
- **Spec ref:** CTAP 2.2 §11.3.1 + ISO 7816-4 §5.1.1.
- **Location:** `src/transport/nfc/channel.rs:147` calling `command::select_file(...)` from the `apdu` crate, which emits `new_with_payload(...)` without an Le.
- **Behavior:** SELECT for AID `A0 00 00 06 47 2F 00 01` is sent as `00 A4 04 00 08 A0 00 00 06 47 2F 00 01` with no trailing Le. FIDO authenticators that strictly enforce Case 4 will reject with SW=6700/6A87. Most authenticators are lenient.
- **Impact:** Interop hazard with stricter authenticator firmwares and chip-on-card implementations.

### `apdu_send` discards SW1/SW2
- **Spec ref:** FIDO U2F NFC §3 (response status word semantics).
- **Location:** `src/transport/nfc/channel.rs:239-246`.
- **Behavior:** `apdu_send` wraps the response in `ApduResponse::new_success(&resp)` and discards the actual SW1/SW2. Non-9000 SWs are surfaced as `NfcError::Apdu(_) → TransportError::InvalidFraming`, losing the distinction between applet-not-selected (6A82), user-presence-required (6985), invalid-key-handle (6A80), etc.
- **Impact:** CTAP1 user-presence retry loop in `proto/ctap1/protocol.rs:111-135` is unreachable on NFC (it relies on `ApduResponseStatus::UserPresenceTestFailed`).

Start in src/transport/nfc/channel.rs at cbor_send, the SELECT call around line 147, and apdu_send around lines 239-246; compare these paths with handle() and the CTAP1 retry loop in proto/ctap1/protocol.rs. Verify the cited CTAP2, ISO 7816-4, and U2F NFC behaviors, then ensure keepalive responses, SELECT Le handling, and original status words work through the existing error and retry paths.

Written by the indexing model from the issue text.