feat!: complete receiver->customer rename (wave 2) + wire-contract CI check

[ericviana]

Held PR — do not merge

This PR is breaking against the API deployed today. It only becomes correct once
blindpay-v2 #1799 reaches production. Labeled hold-until-1799-deploy; the orchestrator
merges after a production probe confirms the cutover.

What breaks

Wave 1 (already shipped, v4.1.0/v4.1.1) aligned the SDK with what production serves
today and added customer.* webhook events plus customer_id list filters, while
explicitly leaving the remaining receiver_* field renames for later because the
deployed API didn't accept them yet. This PR does that remaining work, matching the
target spec (post-#1799).

Migration table

Before	After	Where
receiver_id	customer_id	Payout, Payin, Transfer, Owner, ListPayinsInput/ListPayoutsInput, terms-of-service initiate(), Stellar/EVM/Solana payout+payin create responses
receiver_local_amount	customer_local_amount	quote responses, Payout
receiver_wallet_address	customer_wallet_address	Transfer, CreateTransferQuoteInput
receiver_network	customer_network	Transfer, CreateTransferQuoteInput
receiver_token	customer_token	Transfer, CreateTransferQuoteInput
receiver_invite_redirect_url	customer_invite_redirect_url	UpdateInstanceInput

Also:

• Owner.receiver_id is removed. Wave 1 made it transitional (both receiver_id?
  and customer_id? optional); the target spec's CustomerOut.owners[] items have no
  required array at all, so Owner.customer_id stays optional and receiver_id is
  dropped.
• receiver.new / receiver.update / receiver.delete removed from WebhookEvents.
  They were marked @deprecated in wave 1 (still firing); they stop firing at the same
  cutover, and the target spec's WebhookEndpointIn.events enum no longer lists them.
• api-keys resource removed (blindpay.instances.apiKeys and its types) — API Keys is
  deliberately absent from the public API reference, so it doesn't belong in the SDK.
• No receivers resource remained to remove (wave 1 already renamed it to customers).

Intentionally unchanged

• receiver_amount (singular) — the receiving-side amount on quotes/payins/payouts/
  transfers — keeps its name everywhere. Verified: still present, unrenamed, in every
  file touched.
• currency_type still accepts "sender" / "receiver".
• re_-prefixed IDs are never rewritten (only the field names pointing at them
  change, never the ID values themselves).
• The otc_only_supported_for_sender_without_cover_fees_and_receiver_with_cover_fees
  slug isn't modeled by this SDK (no error-code enum here), so there's nothing to
  preserve — noted for completeness.

Contract check (new CI gate)

Added scripts/contract-check.mjs (dependency-free, node scripts/contract-check.mjs /
bun run contract-check) and wired it into CI as its own job (contract-check.yaml,
parallel to lint). It checks the committed spec snapshot
(.api-sync/spec-snapshot.json, copied from the target spec) against every SDK type
declaration under src/resources/** and types/index.d.ts:

• Hard fail — any field name the SDK declares that doesn't exist as a property (or
  parameter) name anywhere in the snapshot. This is exactly the bug class from wave 2:
  a renamed field left behind. Verified locally by reverting one rename
  (Payout.customer_id back to receiver_id) — the check fails with the exact
  file:line:field. Restored, and it's green again.
• Hard fail — any webhook event enum member in the snapshot that the SDK's
  WebhookEvents union doesn't model (mapped explicitly against
  WebhookEndpointIn.events).
• Warning only (non-blocking) — spec field names the SDK doesn't model anywhere
  (expected: lots of internal/ops-only and provider-specific fields this SDK
  intentionally doesn't surface).

.api-sync/contract-check-allowlist.json holds 22 entries for genuine pre-existing
divergences the check surfaced that are unrelated to this rename and out of scope here
(stale tracking_partner_fee, legacy billing_fee vs billing_fee_amount, partner-fee
evm_wallet_address/stellar_wallet_address that no longer exist on the spec, a couple
of bank-account field-name mismatches, and one camelCase path-only parameter). Each entry
has a reason and an owner; none of them touch a wave-2 field.

Versioning

Major bump: 5.0.0 (breaking field renames). Bumped package.json directly, matching
this repo's convention (the publish workflow reads package.json's version and skips
if already published — no separate changeset version step). Added
.changeset/customers-wave-2.md for the changelog.

Verification

Ran this repo's actual CI commands locally, in the order lint.yaml /
type-checking.yaml / tests.yaml run them, plus the new contract-check job:

bun install --frozen-lockfile
bun run lint:check      # 0 errors
bun run check-types     # 0 errors
bun run test            # 93 passed (19 files)
node scripts/contract-check.mjs   # OK, 1428 fields checked, 23 webhook events checked
bun run build           # builds clean (dist/ not committed, already gitignored)

https://claude.ai/code/session_01F1stiNzuNtJXoXtiW9ZCbs

[BernardoSM]

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