feat(nut18): add PaymentRequestBuilder

[robwoodgate]

Stacked on #683 (based on its head branch); the diff shows that PR's commits until it merges.
Last #683 commit is 4144898

Summary

Adds a fluent builder for authoring NUT-18 payment requests, complementing the PaymentRequestOptions constructor from #683. The builder handles the fiddly parts an options object cannot: transport tag formats, NUT-10 lock serialization, mint URL normalization, and cross-field validation.

const pr = PaymentRequest.builder()
  .amount(100, 'sat') // unit required with amount (NUT-18 MUST)
  .description('Coffee')
  .addMint('https://my.mint') // normalized + deduped
  .mintsPreferred()
  .addNostrTransport(nprofile) // NIP-17 tags applied
  .addHttpPostTransport('https://pay.example.com')
  .addSupportedMethod('bolt12', 5)
  .lock(new P2PKBuilder().addLockPubkey(pk).toOptions())
  .build();

Design notes

• Authoring-strict, wire-lenient. Setters collect state in any order; build() is the single validation point (mp without mints, duplicate sm methods). The PaymentRequest class itself stays lenient since it is also the decode type for foreign requests, whose round-trip must be preserved. Same split as P2PKOptions (data) vs P2PKBuilder.toOptions() (validated authoring).
• lock() closes the receiver-side NUT-10 gap. There was a parse path (toP2PKOptions()) but no build path; receivers hand-serialized NUT-11 tags. The new p2pkOptionsToPRNut10() serializes a P2PKOptions into the request's nut10 option, and the NUT-11 tag serialization moves out of OutputData.createSingleP2PKData into a shared buildP2PKTags() so authored requests and sender-side secrets cannot drift. A round-trip test pins lock() against toP2PKOptions(). blindKeys throws (P2BK blinding is per output at send time). For NUT-10 kinds beyond P2PK/HTLC, nut10() passes a raw option through.
• amount(value, unit) takes the unit as a required parameter, making "amount without unit" unrepresentable (NUT-18: u MUST be set when a is set). unit() remains for amountless requests.
• addMint runs normalizeUrl (the same canonicalization Mint applies) so m entries byte-match what senders compare against.
• Colocation. PaymentRequestBuilder lives in PaymentRequest.ts: a separate file would make static PaymentRequest.builder() a circular import.
• RESERVED_P2PK_TAGS folded into P2PK_KNOWN_TAG_KEYS (identical contents, one set with both meanings documented); tag helpers moved to a new "Lock Tag Serialization" section in NUT11.ts.

Reviewer notes

• The OutputData refactor is behaviour-preserving: buildP2PKTags is the same logic extracted verbatim (threshold gating, post-blinding key layout, tag order), covered by the untouched OutputData tests.
• Usage guide updated (docs-src/usage/payment_requests.md); API report regenerated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.52%. Comparing base (65cb30e) to head (9cb626a).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #780       +/-   ##
===========================================
+ Coverage   39.13%   95.52%   +56.38%     
===========================================
  Files          55       55               
  Lines        5358     5428       +70     
  Branches     1328     1345       +17     
===========================================
+ Hits         2097     5185     +3088     
+ Misses       2794      107     -2687     
+ Partials      467      136      -331     

Flag	Coverage Δ
integration	38.66% <14.44%> (-0.47%)	⬇️

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