oops, just return a fake signature

apps/img/README.md

@@ -13,25 +13,31 @@ key:
   back to the bundled `static/DefaultProfileImage.jpg` asset (served `200` via
   the `ASSETS` binding), so clients always get a valid image. The fallback also
   honours `?sig=p1` and returns a `Content-Signature` header.
-- `GET /<key>?sig=p1` — same, but the response body is RSA-SHA1 signed and the
-  signature returned in a `Content-Signature: key-id=KEY:RSA:p1.rec.net; data=<base64>`
-  header. The client uses this to verify image integrity. **Off by default** —
-  see below.
+- `GET /<key>?sig=p1` — same, plus a
+  `Content-Signature: key-id=KEY:RSA:p1.rec.net; data=<base64>` header. By default
+  that value is a **placeholder, not a real signature** — see below.
 
 The bucket is bound in the Worker as `env.IMAGES` (see `wrangler.jsonc`).
 
 ## Response signing
 
-Signing is behind the `IMG_SIGNING_ENABLED` var in `wrangler.jsonc`, and it is
-**`false`**. It is the worker's dominant CPU cost: signing has to buffer the
-whole object into the isolate rather than streaming it out of R2, then hash the
-full body with SHA-1 and run an RSA-2048 private-key operation — on every request
-the edge cache misses. Nothing verifies the header today, so with the flag off
-`?sig=p1` is accepted and ignored, and untransformed images stream through
-untouched.
+The client requires a `Content-Signature` header to be present when it asks for
+`?sig=p1`, but it never verifies the value. Signing for real is this worker's
+dominant CPU cost: it has to buffer the whole object into the isolate rather than
+streaming it out of R2, then hash the full body with SHA-1 and run an RSA-2048
+private-key operation — on every request the edge cache misses.
 
-Flip the var to `true` to turn it back on. (Resizes still buffer regardless — the
-Photon codec needs the whole image.)
+So by default (`IMG_SIGNING_ENABLED: false` in `wrangler.jsonc`) the header is
+filled with a placeholder derived from the object key: FNV-1a seeds an xorshift32
+PRNG that emits 256 bytes, the length of a real RSA-2048 signature, so the value
+is structurally indistinguishable to the client's parser and stable for a given
+key. It costs no body access, so untransformed images keep streaming.
+
+Set the var to `true` for genuine RSA-SHA1 signatures over the returned bytes.
+Note this is a **placeholder, not a downgrade of a security control** — nothing
+in the system authenticates images either way. Turn it on before relying on the
+header for integrity. (Resizes buffer regardless — the Photon codec needs the
+whole image.)
 
 ### Signing key