js: exclude payer-proof data-range TLVs from invoice reconstruction

[vincenzopalazzo]

Summary

• The TS payer-proof reader treated any non-signature-range TLV as an included invoice field, so an unknown TLV in the payer-proof data range (1001..=999_999_999) was mis-counted as an invoice merkle leaf and the proof was wrongly rejected on the proof_leaf_hashes count check.
• Fix: a record is a genuine invoice field only when type < 240 (normal) or type >= 1_000_000_000 (experimental). This mirrors the rust-lightning reference (tlv_stream_iter), which strips both the signature range (240..=1000) and the data range (1001..=999_999_999) when reconstructing the invoice merkle root.
• Effect: the reader now tolerates ignorable proof extensions (an unknown odd data-range TLV the writer signed over) instead of rejecting them — matching the reference impl in [RFC] Add BOLT 12 payer proof primitives lightningdevkit/rust-lightning#4297.

Review Notes

• Production-safety review passed (2 rounds). The only types whose classification changed are [1006, 999_999_999]; no valid invoice field can live there per the invoice writer rule, and all 5 valid spec vectors + 23 invalid spec vectors still pass.
• Not a forgery vector: proof_signature commits to everything outside 240..=1000 (data-range TLVs included), so an injected data-range TLV invalidates proof_signature unless the payer signed it. The invoice signature is reconstructed over invoice fields only, matching the issuer's original root.
• Adds a regression test that signs a proof carrying an unknown odd data-range TLV (1007) and asserts it still verifies. It fails before the fix (leaf_hashes count (11) must match included non-signature TLV count (12)).

Decision Log

Hardest decision: whether to silently exclude unknown data-range TLVs (as done here, matching rust tlv_stream_iter) versus rejecting them. I went with exclude-and-tolerate because the reference does, and because proof_signature already commits to them so toleration is safe. The alternative — full BOLT TLV even/odd "it's ok to be odd" enforcement on the proof stream — is a broader, library-wide change (js/src/tlv.ts) that this repo doesn't do anywhere yet, so I kept it out of scope.

Alternatives rejected:

• Reject any unknown data-range TLV: diverges from the rust reference and breaks forward-compat for ignorable extensions.
• Hardcode the data-range bounds inline at the call site: less readable; extracted isIncludedInvoiceType instead to document the rule once.

Least confident about: the even/odd TLV semantics gap. An unknown even data-range TLV is silently excluded here but rust's typed TLV stream would reject it as an unknown mandatory field. That is a pre-existing, library-wide TLV-handling difference (the TS lib never enforces even/odd mandatory rules), not specific to payer proofs, so I left it for a follow-up. Happy to revisit if you want it enforced here.

Test plan

• npx tsc --noEmit clean
• npm test — offers 53, payer-proof 35 (was 34), generated 22, verify 3
• Regression test fails without the fix, passes with it

[vincenzopalazzo]

Self-review: I found one spec-compliance issue in payer-proof TLV parsing. The inline comment captures the specific concern.