Add bindings/c-ffi: C / C++ bindings alongside uniffi

[Jainakin]

Summary

Adds a new bindings/c-ffi/ sub-crate that exposes the SdkNode API over a
C ABI. It is a thin extern-"C" shim on top of the existing UniFFI-exposed API
in src/uniffi_api/ — same async-bridge (block_on_sdk in
src/uniffi_api/state.rs), same SdkNode struct, no changes to src/.

This unblocks consumers that don't have a UniFFI runtime: Node.js / Bun via
N-API, Bare native addons,
Python via ctypes, etc. The intended first consumer is a bare-runtime
native addon mirroring the rgb-lib-bare
pattern.

What's exposed

61 extern "C" functions covering the full SdkNode surface plus lifecycle,
namespace-level helpers, and dropping primitives:

• Lifecycle (5): rln_sdk_node_new, rln_sdk_node_init,
  rln_sdk_node_unlock, rln_sdk_node_shutdown, free_sdk_node
• Channels / peers (7): rln_open_channel, rln_close_channel,
  rln_list_channels, rln_get_channel_id, rln_connect_peer,
  rln_disconnect_peer, rln_list_peers
• LN payments / invoices (10): rln_send_payment, rln_keysend,
  rln_ln_invoice, rln_invoice_status, rln_cancel_hodl_invoice,
  rln_claim_hodl_invoice, rln_decode_ln_invoice, rln_decode_rgb_invoice,
  rln_get_payment, rln_list_payments
• RGB (10): rln_rgb_invoice, rln_send_rgb, rln_refresh_transfers,
  rln_fail_transfers, rln_inflate, rln_list_transfers,
  rln_list_unspents, rln_post_asset_media, rln_get_asset_media,
  rln_check_proxy_endpoint
• Asset issuance / metadata (7): rln_issue_asset_{nia,cfa,ifa,uda},
  rln_list_assets, rln_asset_balance, rln_asset_metadata
• BTC / node info / utxos / sync (11): rln_node_info, rln_network_info,
  rln_address, rln_btc_balance, rln_sign_message, rln_estimate_fee,
  rln_check_indexer_url, rln_send_btc, rln_create_utxos,
  rln_list_transactions, rln_sync
• Swaps / onion (6): rln_maker_init, rln_maker_execute, rln_taker,
  rln_send_onion_message, rln_get_swap, rln_list_swaps
• Module-level helpers (4): rln_uniffi_healthcheck,
  rln_uniffi_is_initialized, rln_sdk_initialize, rln_sdk_shutdown
• String free (1): rln_free_string

ABI / wire format

Mirrors the
rgb-lib c-ffi pattern:

• COpaqueStruct opaque handle for SdkNode.
• CResult / CResultString tagged-union returns; on Err, inner carries
  the formatted error message — caller frees via rln_free_string.
• All complex inputs and outputs cross the boundary as JSON strings.
  Typed wrappers (PublicKey, Txid, ContractId, ChannelId,
  PaymentHash, Bolt11Invoice) are passed as their canonical string forms
  (hex / bech32 / BIP-21) inside the JSON or as bare string args.
• cbindgen generates rln.h and rln.hpp at build time. Both are
  committed to keep first-time consumers (and the included example.c)
  buildable without running build.rs first.

JSON-friendly mirror request/response types live in src/json_types.rs
(~1900 LoC, all mechanical) with TryFrom / From conversions to and from
the UniFFI typed wrappers. This avoids requiring serde::{Serialize, Deserialize} derives on the existing types in src/uniffi_api/types.rs.

File layout

```
bindings/c-ffi/
Cargo.toml # rln-c-ffi crate, [workspace] block, [patch.crates-io]
Cargo.lock # committed — pins rgb-lib to the same rev as parent
build.rs # cbindgen → rln.h / rln.hpp
cbindgen.toml
Makefile # rust-build + example wiring (mirrors rgb-lib c-ffi)
README.md
rln.h # generated, committed
rln.hpp # generated, committed
example.c # smoke test (healthcheck + is_initialized)
src/
lib.rs # 61 extern "C" wrappers
api.rs # handler functions (parse JSON → call SdkNode → JSON)
json_types.rs # mirror request/response structs + converters
utils.rs # COpaqueStruct, CResult helpers, ptr helpers
```

No changes to existing files outside bindings/c-ffi/.

Build & verify

```sh
cd bindings/c-ffi
cargo build # libs + headers
make build-example # gcc -lrlncffi
DYLD_LIBRARY_PATH=target/debug ./example

rgb-lightning-node c-ffi smoke test

[healthcheck] OK: rgb_lightning_node_uniffi_ready

[is_initialized] OK: false

```

Outputs: target/debug/librlncffi.{a,dylib} (≈170 MB dylib, ≈1.6 GB static
unstripped on darwin; release builds are far smaller).

Notes on workspace / dep resolution

The c-ffi sub-crate is its own Cargo workspace ([workspace] block). It
mirrors the parent crate's [patch.crates-io] table to keep lightning /
lightning-background-processor unified onto the in-tree rust-lightning
submodule — without that, registry pulls of lightning-net-tokio etc.
trigger trait-bound mismatches with the path-deped lightning. The
committed Cargo.lock pins rgb-lib to the same revision the parent
Cargo.lock uses, so a fresh clone + cargo build --locked reproduces
without cargo update --precise gymnastics.

A direct lightning / lightning-invoice dep is added so the sub-crate
can construct PaymentHash / ChannelId by their real crate paths
(rgb_lightning_node::PaymentHash is a type alias; Rust 2021 doesn't allow
tuple-struct construction through aliases).

Out of scope

• Prebuilt static libs / .so / .dylib distribution (will live in the
  consuming rgb-lightning-node-bare package).
• VSS / backup helpers — the existing UniFFI surface doesn't expose them on
  SdkNode; can be added later if/when the SDK does.
• Async / event-stream callbacks. The SdkNode surface is sync today; HOLD
  invoice notifications etc. are not exposed by UniFFI either.