Add a BlindedMessagePath constructor for external recipients

[dunxen]

We're currently working on BOLT 12 receives in Bark and, at the moment, we run a CLN node for
handling lightning sends and receives atomically for clients of our ark server.

For BOLT 12 receives, we will allow clients to create their own offers (with their own per-offer derived issuer_signing_pubkey), and register them with the server. Requests for invoices on that offer are forwarded to clients who generate and sign invoices themselves. The CLN node is still the final recipient and introduction node on the lightning side. The rest of the flow is similar to our atomic BOLT 11 hold invoice/HTLC VTXO flow.

All our client lightning code uses LDK and we need a way to construct a recipient hop that CLN actually understands with no authentication. Deanonymisation protections are irrelevant in this scenario.

I'm open to ideas if you'd prefer not to have such a constructor just to cover our case as it is somewhat new.

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[ldk-claude-review-bot]

The code is consistent with my prior review. The implementation is symmetric and correct, and the changelog now documents both new APIs (resolving my prior comment).

No issues found.

The previously-flagged changelog issue (missing BlindedMessagePath::new_for_external_recipient entry) is now resolved — pending_changelog/blinded-path-external-recipient.txt documents both the message-path and payment-path constructors. The code changes remain correct: new_for_external_recipient passes None for both local_node_receive_key and context, all existing callers correctly wrap their values in Some(...), and the dropped deanonymization protection is intentional and documented.

[codecov]

Codecov Report

❌ Patch coverage is 99.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.29%. Comparing base (1c1a4ad) to head (d593175).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/blinded_path/payment.rs	98.38%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4742      +/-   ##
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- Coverage   86.90%   86.29%   -0.61%     
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  Files         161      160       -1     
  Lines      111680   111615      -65     
  Branches   111680   111615      -65     
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- Hits        97052    96315     -737     
- Misses      12115    12660     +545     
- Partials     2513     2640     +127     

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tests	86.29% <99.00%> (+0.04%)	⬆️

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[dunxen]

Putting this in draft. Also need a similar constructor for BlindedPaymentPath. We were on an older version of LDK so didn't pick that up.