feat: BRC-138 single-use signed proofs (login in one request)

[kryp2]

Agent: dsh-peck/deepseek-v4-flash

Summary

Modernization round for bsv-brc-python against the 2026-08 BRC ecosystem
(spec set current to 2026-08-12, 150+ standards): the first Python
implementation of BRC-138 — Single-Use Signed Proofs for Request
Authentication, proven byte-compatible with the canonical TypeScript
reference @bsv/auth.

What's included

• bsv_brc.brc138 — create_auth_proof / verify_auth_proof /
  check_auth_proof_data over the spec's canonical encoding, BRC-42/43
  child-key derivation per proof (protocolID [2, name], keyID = nonce,
  counterparty = verifier), SHA-256 + DER ECDSA signing (matches
  @bsv/sdk createSignature). Wire form per spec (signature = byte
  values; hex accepted for wallet interop). Optional request-payload
  binding (VarInt length-prefixed).
• Single-use stores — SingleUseStore ABC, MemorySingleUseStore
  (locked, lazy eviction), SqliteSingleUseStore (atomic
  INSERT OR IGNORE, bounded retention).
• AuthProofMiddleware — optional Starlette/ASGI auth gate
  (x-bsv-auth-proof header or JSON body proof member).
• crypto.keys.derive_signing_public_key — verifier-side BRC-43
  derivation (inverse of derive_signing_key).
• Interop proof — examples/brc138_interop/ live 4-way check against
  @bsv/auth + @bsv/sdk (Python↔Node, bodyless + body-bound), plus
  pinned certified vectors in tests/test_brc138_interop.py (Node-free
  regression).
• docs/MODERNIZATION.md — full gap analysis vs the current BRC set:
  402 payment family (BRC-118 multipart, BRC-120 x402, BRC-121 simple 402),
  BEEF V2 (BRC-96/158), overlay sync (BRC-76 GASP, BRC-136 BASM), BRC-35;
  corrects the stale/factually-wrong "out of scope" claims in README/CHANGELOG
  (BRC-101 is SHIP/SLAP facilitator URLs, BRC-108 is the Identity-Linked
  Token Protocol, BRC-116 is Wallet Permissions — nothing sCrypt).

Verification

• Full suite: 235 tests pass (197 pre-existing + 38 new) on Python 3.14
  with latest bsv-sdk 2.3.3 (pin floor was 2.1.3 — suite was already
  green on the newer SDK).
• Live cross-implementation check against @bsv/auth (Node): all 4
  directions pass.

Notes for review

• Branch: feat/brc138-auth-proofs-2026-08-17
• The examples/brc138_interop/ package-lock.json pins @bsv/auth
  0.1.3 / @bsv/sdk 2.4.1 for reproducible interop runs; node_modules/
  is gitignored.
• Default protocol [2, "bsv auth proof"] matches @bsv/auth so proofs
  interop across languages out of the box.