feat: expose custom transport API via from_transport()

[frijolo]

Summary

Adds a public BitBox::from_transport() constructor that accepts any communication::ReadWrite implementation and performs the U2F-HID framing internally. This lets callers delegate raw USB I/O to platform-specific code, instead of relying on hidapi.

Two small changes:

• Make the communication module public, exposing the ReadWrite trait and Error.
• Remove the feature gate on FIRMWARE_CMD so from_transport (available without the usb/wasm/simulator features) can perform the framing.

Motivation

We want to drive a BitBox02 from Android, where USB access must go through the Java UsbManager API and hidapi is not available. The existing constructors (from_hid_device, usb::get_any_bitbox02) all assume hidapi, so there is currently no way to connect on such platforms.

With from_transport(), a caller implements only the ReadWrite trait over their platform's transport (on Android, a channel bridged to the Java USB stack) and gets a fully functional BitBox — the U2F-HID framing is handled by the library, exactly as it is for the USB path. Desktop/hidapi callers are unaffected.

This mirrors how the official BitBoxApp already drives the device on Android: its GoViewModel.java opens the device through UsbManager/UsbDeviceConnection, performs the bulkTransfer() I/O in Java, and hands a ReadWriteCloser to the Go backend (no hidapi on Android). from_transport() is simply the Rust/bitbox-api equivalent of that pattern.

What changed

• src/lib.rs: new pub async fn from_transport(transport: Box<dyn communication::ReadWrite>, noise_config: Box<dyn NoiseConfig>) -> Result<BitBox<R>, Error>; mod communication → pub mod communication.
• src/communication.rs: FIRMWARE_CMD is no longer gated behind usb/wasm/simulator, since from_transport needs it without those features.

No existing API changes; this is purely additive.

Testing

• Builds with and without the usb feature.
• Downstream: an Android app connects to and signs with a physical BitBox02 through a channel-based ReadWrite transport built on from_transport() — pairing, xpub export, wallet/policy registration, and PSBT signing (singlesig + multisig) all verified on-device.

[benma]

Could reuse Self::from_transport here and in from_simulator below

[benma]

How about pub use comminucation::ReadWrite instead, to keep the public API surface smaller?

[frijolo]

Applied both suggestions:

• from_hid_device/from_simulator now reuse from_transport() internally.
• communication module stays private; re-exporting ReadWrite (and Error,
  which its methods return) from the crate root instead.

Built and verified successfully in our downstream app on both desktop
(Linux) and Android — the latter being what exercises from_transport(),
the core of this PR. Full on-device BitBox02 validation is still
pending on my end (no physical hardware until next week); will follow
up once that's done.

[benma]

Applied both suggestions:

* from_hid_device/from_simulator now reuse from_transport() internally.

* communication module stays private; re-exporting ReadWrite (and Error,
  which its methods return) from the crate root instead.

Built and verified successfully in our downstream app on both desktop (Linux) and Android — the latter being what exercises from_transport(), the core of this PR. Full on-device BitBox02 validation is still pending on my end (no physical hardware until next week); will follow up once that's done.

@frijolo I squashed your two commits, added a version-bump commit and published this change as v0.13.0.

[frijolo]

Thanks for the quick review and for cutting the release!