feat(sep12): SEP-12 PUT/GET /customer client with CustomerStatus types

[Johnalex-hub]

Summary

Implements B013 — adds a minimal SEP-12 /customer HTTP client to lib/stellar/sep12.ts and required types to types/index.ts.

• putCustomer(kycServer, jwt, fields) — PUT /customer with JSON body and Bearer auth
• getCustomer(kycServer, jwt, id?) — GET /customer with optional id query param
• CustomerStatus type: ACCEPTED | NEEDS_INFO | PROCESSING | REJECTED
• Sep12CustomerResponse and Sep12CustomerField interfaces in types/index.ts
• Unknown anchor status strings normalize to NEEDS_INFO, matching SEP-24 status-map convention

Test plan

• putCustomer sends correct method/headers/body, returns normalized response
• getCustomer sends correct URL, appends id param when provided
• Unknown status normalizes to NEEDS_INFO
• Non-ok responses throw via parseSepErrorBody
• 12 tests pass (npx vitest run tests/sep12.spec.ts)

Closes #446

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[ezedike-evan]

Two CI failures to fix before this can merge:

1. Prettier formatting — check (node 20) fails at the "Prettier formatting" step. Run:

   npx prettier --write lib/stellar/sep12.ts tests/sep12.spec.ts types/index.ts
   

2. Commitlint — the commit subject line exceeds the 72-character limit. Please shorten it, e.g.:

   feat(sep12): add putCustomer/getCustomer client with CustomerStatus types
   

   (Trim it so the header is ≤72 chars, then amend: git commit --amend --no-edit after fixing the message, and force-push.)