Problem

`ALBProcessor.getHeaders` (`src/adapter/aws-lambda/handler.ts:511-518`) joined every multi-valued header with `'; '`:

```ts
if (event.multiValueHeaders) {
for (const [key, values] of Object.entries(event.multiValueHeaders)) {
if (values && Array.isArray(values)) {
const sanitizedValue = sanitizeHeaderValue(values.join('; '))
headers.set(key, sanitizedValue)
}
}
}
```

RFC 9110 §5.3 requires `, ` as the separator when combining field-line values; `; ` is only correct for the `Cookie` request header (RFC 6265 §5.4) and a handful of structured headers using parameters.

A real client sending:

```
Accept: text/html
Accept: application/json
```

is delivered to ALB as `multiValueHeaders: { accept: ['text/html', 'application/json'] }` and reaches the Hono handler as the single value `text/html; application/json`. Content-negotiation libraries (and Hono's own `c.req.header('accept')` consumers) interpret that as one media range with a parameter, not two distinct ranges. The same flaw affects `Cache-Control`, `Forwarded`, `If-None-Match`, `Accept-Language`, `Vary`, `Link`, etc.

Fix

Append each value individually so the WHATWG `Headers` combine them with `, ` automatically. Special-case `cookie` (case-insensitive) to keep the `; ` join, matching the existing test expectation and RFC 6265.

```ts
if (key.toLowerCase() === 'cookie') {
headers.set(key, sanitizeHeaderValue(values.join('; ')))
} else {
for (const value of values) {
headers.append(key, sanitizeHeaderValue(value))
}
}
```

Test

Added two tests under `EventProcessor.createRequest > ALBProcessor multi-value header handling`:

1. Non-cookie multi-value — sends two `Accept` values plus two `Cache-Control` values via `multiValueHeaders`, asserts `request.headers.get('accept')` returns `'text/html, application/json'` and `cache-control` returns `'no-cache, no-store'`. Fails on `main` (returns `'text/html; application/json'`); passes with the fix.

2. Cookie still uses `; ` — confirms the special case continues to behave correctly per RFC 6265.

• Add tests
• Run tests (`bun run test` — 31/31 passes)
• `bun run format:fix && bun run lint:fix`
• Add TSDoc/JSDoc — no API surface change