docs: fix interface drift across adapters.md, better-auth-resend, react-email

[ssilvius]

Summary

Second pass of doc accuracy fixes after reading the 967-line `packages/core/docs/adapters.md` top to bottom and cross-referencing every claim against the actual source. Found significant drift: invented exports, wrong factory names, wrong field names, wrong interface shapes, wrong config schemas.

Some of the same errors existed in docs I introduced in PR #76 (now merged to main) -- fixing those here too rather than opening a separate PR.

The errors

adapters.md -- Resend section

• `new ResendService + new ResendProvider` -> `createResendProvider({ apiKey, fromEmail })`. `ResendProvider` is not an exported class.
• `CampaignParams.audienceId` -> `listId` (real schema field name).
• `new MockEmailProvider()` -> `createMockEmailProvider()`. Mock is a factory too.
• Documented the real mock surface: `sentEmails` top-level property, `getState()`, `clear()`.

adapters.md -- Cloudflare section

• `handleInboundEmail` and `R2BlobStorage` class referenced throughout -- neither exists. Replaced with the real building blocks: `createR2Storage`, `parseEmailHeaders`, `hashContent`.
• `BlobStorage` interface shape wrong: had `putRaw` / `putText` / `putHtml` three-method split. Real interface is single `put(key, content, options?)`, `get(key, options?)`, `delete(key)`, `generateKey(contentHash, extension)`.
• Worker entry point example rewritten to show the compose-your-own pattern: read bytes -> parse headers -> hash -> store.
• Thread matching: clarified that it is the consumer's responsibility, not a shipped feature of the adapter. The adapter provides parsing building blocks; consumer writes the matching logic.
• `InMemoryBlobStorage` example rewritten as a factory closure returning `BlobStorage`.

adapters.md -- Workers AI section

• `ClassifyEmailWorkflow` and `handleEmailClassifyQueue` referenced (with `/workflow` and `/queue` subpath imports) -- neither exists. The package does not ship those subpaths.
• `createEmailClassifier` -> `createWorkersAIClassifier` (real name).
• Queue consumer example rewritten to show consumer-implemented queue handling using the real classifier.
• Mock AI type is `AiBinding` from the package, not `Ai`.

adapters.md -- React Email section

• Import paths: `BaseEmail` and `OtpEmail` were imported from the package root, which would fail at build time. Real exports are at `/templates` and `/otp` subpaths respectively.
• Rendering: rewrote to show the registry pattern. The renderer is name-keyed, not component-keyed. Templates register by string name (`createReactEmailRenderer({ otp: OtpEmail })` or `renderer.register("name", Component)`) and `render(name, props)` looks up by name. The doc had `render(Component(props))` which neither matches the interface nor is how the renderer works.

adapters.md -- better-auth-resend section

• `resendOTP` config was entirely wrong. Doc claimed `resendOTP(env, options?)` with `env = { RESEND_API_KEY, FROM_EMAIL }`. Real shape is a single `ResendOTPConfig` object: `{ apiKey, fromEmail, brandName, logoUrl?, websiteUrl?, expiryMinutes?, baseUrl? }`. Rewrote the integration example and documented the full config.

adapters.md -- Custom adapter example

• Converted the `class PostmarkProvider implements EmailProvider` example to a `createPostmarkProvider` factory closure. Matches the monorepo factory-pattern rule in CLAUDE.md.

adapters.md -- Barrel exports section

• Fixed 4 wrong imports (`ResendProvider`, `R2BlobStorage`, `handleEmailClassifyQueue`, `ClassifyEmailWorkflow`) that listed non-existent exports. Replaced with the real exports via their correct subpaths.

better-auth-resend README + docs/usage.md (drift I introduced in #76)

• `resendOTP` config shape: was `{ apiKey, from, appName }`, real is `{ apiKey, fromEmail, brandName, ... }`. Fixed example and config table. Added the 4 optional fields (`logoUrl`, `websiteUrl`, `expiryMinutes`, `baseUrl`) that the real schema exposes.
• "What the user sees" subject template updated: `" verification code: "`, not `"Your code"`.
• "Replacing the template" example now uses the registry pattern + `provider.sendEmail` (not `.send`).

react-email README + docs/templates.md (drift I introduced in #76)

• Renderer: registry pattern. `createReactEmailRenderer({ otp: OtpEmail })` then `renderer.render("otp", { code, brandName })`. The previous example called `render(OtpEmail({ otp, appName }))` which neither matches the interface nor uses real prop names.
• OtpEmail props: `otp` -> `code`, `appName` -> `brandName`, `expiresInMinutes` -> `expiryMinutes`.
• "Writing your own template" example rewritten to register the custom template with the renderer.

Why these slipped past the previous accuracy sweep

PR #80 fixed schema drift that @vault-2026 had already flagged (22 items + 3 misleading in `core-reference.md`). That review focused on the schema sections of the core doc. The 967-line `adapters.md` and the per-package usage docs were never audited. This PR closes that gap.

Verification

• `pnpm test`: 609 passing across 37 files (no code changes, same count)
• `pnpm typecheck`: clean
• `pnpm lint`: 0 warnings, 0 errors
• `pnpm format:check`: clean on all modified files
• `legion-simplify` gate: clean
• Every code example cross-referenced against the actual TypeScript source (not the README claims, not the Zod schemas alone -- the implementation files)

Scope

Single commit, 5 files, all documentation. No runtime behavior change.