feat: web browsing on canvas via pap://domain.com

[toadkicker]

Summary

• pap_uri.rs: Split is_registry_host into is_local_registry (localhost/IPv4/IPv6 → Registry) and is_web_domain (dotted FQDN → HttpsEndpoint). pap://ebay.com now resolves to https://ebay.com instead of misrouting to a registry peer. Registries are federated and announced at handshake time — not addressed via the URI scheme.
• templates.rs: Add WebPageTemplate — renders schema:WebPage JSON-LD as a browser-tab-style canvas block with URL bar (pap:// link for in-canvas navigation), title, byline, description, and body text extract.
• mod.rs: Register WebPageTemplate in create_default_registry().
• topbar.rs: pap:// suggestion dropdown now shows Browse pap://domain.com for web domains and auto-submits on selection; catalog agent completions unchanged.

End-to-end flow

pap://ebay.com  →  HttpsEndpoint("https://ebay.com")
  →  detect_intent: starts_with "https://"  →  Web Page Reader
  →  GET https://ebay.com  →  schema:WebPage JSON-LD
  →  WebPageTemplate.render()  →  browser-tab block on canvas
  →  links in block  →  pap://ebay.com/...  →  next browse block

Test plan

• cargo test -p papillon-shared — 41 pap_uri tests green including bare_domain_browse, domain_with_path_browse, domain_with_port_browse, localhost_is_local_registry, localhost_with_port_is_local_registry
• pap://ebay.com in address bar → canvas block with title, description, body text
• pap://localhost:8080 → still routes to Registry (no regression)
• https://ebay.com typed directly → still works
• Clicking URL in WebPage block → new browse block

🤖 Generated with Claude Code

[github-actions]

Benchmark Regression Report

PAP Protocol Benchmark Regression Check
========================================
Baseline: .bench-baseline/baseline.json
Threshold: 30%

  ed25519_keypair_generation                19.7 µs  (baseline: 19.7 µs, -0.3%)  [ok]
  did_key_derivation                         1.5 µs  (baseline: 1.5 µs, +0.0%)  [ok]
  mandate_create_sign                       23.9 µs  (baseline: 23.9 µs, -0.1%)  [ok]
  mandate_chain_verify_depth3              125.5 µs  (baseline: 126.9 µs, -1.1%)  [ok]
  sd_jwt_issue_5claims                      27.9 µs  (baseline: 27.9 µs, +0.0%)  [ok]
  sd_jwt_verify_disclose_3of5               44.5 µs  (baseline: 44.3 µs, +0.6%)  [ok]
  session_open_full_lifecycle              108.4 µs  (baseline: 108.7 µs, -0.3%)  [ok]
  receipt_create_cosign                     47.8 µs  (baseline: 48.9 µs, -2.2%)  [ok]
  federation_announce_local                 56.0 µs  (baseline: 56.0 µs, +0.0%)  [ok]

All benchmarks within 30% of baseline.

_{Threshold: 10% regression vs baseline from main}