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/src/capnweb-browser.ts

@@ -0,0 +1,10 @@
+/**
+ * @file capnweb's browser build (a self-contained ES module), as text, for
+ * the worker to serve at /_capnweb.js; wrangler's Text rule for this path
+ * makes the import a string. Self-hosted: no third-party script on a page
+ * that holds a bearer key.
+ */
+// @ts-expect-error: a Text-rule import (wrangler.jsonc "rules"), not code
+import source from "../node_modules/capnweb/dist/index.js";
+
+export const CAPNWEB_BROWSER_JS: string = source;