fix(http): raw writeHead routes lost every header the security hook set

`reply.raw.writeHead()` writes straight to the Node response and bypasses
Fastify's header store, so everything the `onRequest` security hook granted is
silently dropped on every route that answers that way.

The visible symptom is CORS. The hook emits `Access-Control-Allow-Origin` for
localhost origins, so a page served from a local dev server may call every `/api`
endpoint cross-origin — except the four below, whose requests fail. The security
headers (`X-Content-Type-Options`, `X-Frame-Options`, CSP) were being lost the
same way.

Affected: `GET /api/events`, and `file-raw` / `tail-file` / `download` in
file-routes.ts. Each now spreads the inherited headers first and lets its own
headers win over them.

Tests drive a real WebServer and compare `/api/events` against `/api/status` for
the same Origin — the point of the fix being that the SSE route stops being the
odd one out. Verified in both directions: with the fix removed, 3 of the 5 fail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.changeset/sse-inherits-security-headers.md

@@ -0,0 +1,16 @@
+---
+'aicodeman': patch
+---
+
+Routes that answer with `reply.raw.writeHead()` no longer drop the headers the
+security hook set.
+
+`writeHead` writes straight to the Node response and bypasses Fastify's header
+store, so everything the `onRequest` hook granted was silently lost — including the
+`Access-Control-Allow-Origin` it emits for localhost origins, and the
+`X-Content-Type-Options` / `X-Frame-Options` / CSP headers. A localhost page could
+therefore call every other `/api` endpoint cross-origin while its EventSource
+failed CORS.
+
+Affects `GET /api/events` and the three raw-writing routes in `file-routes.ts`
+(`file-raw`, `tail-file`, `download`).