[Bccorb]

Closes #463.

What

• POST /oauth/:providerId/start returns a random binding (32 bytes, base64url) next to state. Only its SHA-256 goes into the signed state; it is never put in the authorization URL.
• POST /oauth/:providerId/callback takes binding and refuses a state whose hash it does not match (constant-time), before the replay cache records the state and before the provider is contacted. A wrong binding therefore cannot use up a state its rightful client has yet to finish.
• Refusals log oauth_login_failed with state_binding_missing or state_binding_mismatch (was a single invalid_state), so an operator can see old clients during the upgrade.
• OAUTH_ALLOW_UNBOUND_STATE=true accepts a callback with no binding, warns at boot, and is removed in v0.20. A binding that is sent is always checked.
• docs/oauth.md and docs/configuration.md describe the binding and the escape hatch.

Contract change

Additive in the schemas, but by default it refuses OAuth callbacks from clients that do not send binding. Patch changeset by maintainer decision (v0.19 ships as 0.19.x); the changeset body states the break.

Ripple:

• @seamless-auth/types (feat(oauth): add the state binding to the start response and callback request seamless-auth-types#104) adds the field. This PR needs that release: with types 0.28.0 the request schema strips binding, so CI fails until the dependency is bumped. Draft until then.
• @seamless-auth/client / react / react-native: keep binding from startOAuthLogin and send it from finishOAuthLogin (PR to follow).
• Server adapters (express, fastify, nextjs, Go, Python, Rust) pass both bodies through, so no code change.
• seamless-cli verify harness posts callbacks directly and needs the field.

Checks (local, against the unreleased types build)

• typecheck, lint, format:check clean; build ok
• npm run test:run: 138 files, 1891 passed, 1 todo
• npm run coverage: all files 98.93 statements / 95.43 branches / 99.29 functions / 99.16 lines