diigo-cache: fill from a webkey page over Cap'n Web

Starting a fill no longer POSTs the cache id to /_fill. Instead:

- the created page links the cache's webkey, /cache/#<cache id>: a
  skeleton page, the same for every key, that reads the key from the
  fragment (never sent to the server or in a Referer)
- its script trades the key in-band for a capability over Cap'n Web at
  /api: CacheApi.cache(key) returns a CacheCap facet (fill, status,
  fingerprint, the public companion's fill) and never the DO's wiring
  methods; malformed and unknown keys get one refusal that never echoes
  the key, and each refusal is logged without it
- then calls startFill(): trade and call pipeline into one round trip
- the worker serves capnweb's browser build itself at /_capnweb.js (a
  wrangler Text rule), so no third-party script sits beside the key

The /_fill route and its forms are gone. Tests run the real capnweb
client against the worker (fetch routed to worker.fetch).

Generated-by: Claude Opus 5.5

diigo-cache/wrangler.tests.jsonc

@@ -5,6 +5,7 @@
 	"name": "diigo-cache-tests",
 	"main": "./src/index.ts",
 	"compatibility_date": "2026-09-29",
+	"rules": [{ "type": "Text", "globs": ["**/capnweb/dist/index.js"], "fallthrough": true }],
 	"services": [
 		{ "binding": "WEBAPP", "service": "fake-webapp", "entrypoint": "WebAppFactory" }
 	],