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/package.json

@@ -13,5 +13,8 @@
 		"typescript": "^5.9.0",
 		"vitest": "4.1.11",
 		"wrangler": "^4.148.0"
+	},
+	"dependencies": {
+		"capnweb": "^0.12.0"
 	}
 }