feat: add configurable route hints to BOLT11 receive APIs

[ajaysehwal]

Summary

• Add data-carrying RouteHints (None / Automatic / Custom { user_channel_ids }) for BOLT11 invoice creation (replaces the earlier RouteHintsMode + optional IDs shape)
• Add receive_with_route_hints, receive_variable_amount_with_route_hints, and the matching *_for_hash_with_route_hints variants (Rust + UniFFI)
• Existing receive* APIs stay on Automatic
• All modes go through LDK ChannelManager::create_bolt11_invoice via route_hints_override (pinned ZeusLN/rust-lightning)

Custom takes up to 3 UserChannelIds and builds hints from ready channels that have an inbound SCID and forwarding info. Invalid or unusable channel IDs fail immediately with InvalidChannelId (they are not skipped).

[kaloudis]

Reviewed the branch at e70a3c6. The core mechanics check out: claimability parameters (payment secret registration, MIN_FINAL_CLTV_EXPIRY_DELTA, currency, MPP feature, signature recovery) match what ChannelManager expects per the pinned rust-lightning, cargo check passes with and without --all-features, and uniffi bindgen generates valid Python/Kotlin/Swift bindings including the None variant.

Five inline comments below. Minor notes not worth separate threads:

• The PR description is stale: it still says unusable channels are skipped, but e70a3c6 made them fail fast.
• Custom-hint validation runs after create_inbound_payment*. No state leaks (inbound payments are stateless), but validating the hints first is the cleaner fail-fast order and skips pointless key derivation on the error path.
• sha256::Hash::from_slice(&payment_hash.0) on a fixed [u8; 32] can never fail — sha256::Hash::from_byte_array(payment_hash.0) removes the dead error branch.
• The *_for_hash receive variants got no route-hints counterpart even though receive_inner now supports the combination — worth deciding whether that asymmetry is intentional before this API ships.

[kaloudis]

Three items before merge:

1. Run cargo fmt. The new code in src/payment/bolt11.rs fails cargo fmt --check (import ordering of bitcoin::hashes::Hash, the RouteHints::Custom match arm, receive_wrapped's signature and several call sites) — this is what's failing the build (…) CI jobs in ~30s. The remaining red checks (check-kotlin, integration tests) are inherited from the zeus base branch and not caused by this PR.

2. Fix the PR description. It still says "Unusable channels are skipped;" — the implemented behavior (correctly) fails fast with InvalidChannelId instead. The doc comments and UDL already say this; only the description contradicts them.

3. (Optional) Copy the cap note to the other method docs. The MAX_CUSTOM_ROUTE_HINTS limit is documented on the enum and on receive_variable_amount_for_hash_with_route_hints, but not on the other three *_with_route_hints methods.