Correct WebCrypto result types and verify key workflows #390

Part of #382 — v1.0.0 alpha readiness.

Problem

Several SubtleCrypto operations are declared as returning JSON.t, although their native results are binary values or opaque keys:

• src/crypto/SubtleCrypto.res:23-L89: signing/digests return ArrayBuffer; verification returns boolean; key generation returns a CryptoKey or key pair; derivation returns CryptoKey.
• src/crypto/SubtleCrypto.res:126-L158: key export depends on format, and key wrapping returns ArrayBuffer.

A native Node WebCrypto probe confirmed the HMAC generate/sign/verify, digest, and raw-export result shapes. Current signatures prevent safe composition of key-producing and key-consuming functions without escaping the types.

Acceptance criteria

• Correct fixed native result types, including verify to boolean.
• Model algorithm-dependent key generation and format-dependent key export with explicit overloads or a decoded abstract type, following existing binding conventions.
• Re-export the public support types required to compose the supported workflows without unsafe casts or internal-module imports.
• Compile and run generate -> sign -> verify, digest -> byte view, and representative raw/JWK export workflows.
• Cover a key-pair generation result as well as a single symmetric key.
• Update affected docs and compile fixtures, with negative coverage for the former incorrect result assumptions.

Do not broaden this issue into an unrelated cryptography abstraction or change native promise rejection semantics.

Coordination

Use the regression infrastructure in #383. Source fixes can be developed in parallel, but this issue should close with executed regression coverage, not only a successful compile.