refactor: BlockUpdate typed events + BlockContext provider pattern

[toadkicker]

Summary

• BlockUpdate — new wire type in papillon-shared that carries only backend-owned fields. linked_block_ids and auto_expand are structurally absent, so the type system prevents events from ever overwriting frontend-only state. All 9 emit sites in commands/canvas.rs migrated from CanvasBlock to BlockUpdate.

• apply_block_event simplified — the save-and-restore pattern is deleted. The new body does direct field assignment; linked_block_ids and auto_expand are never in scope. New frontend-only fields added to CanvasBlock in future are automatically safe without touching this function.

• BlockContext provider — replaces SourceBlockId(pub String) with a #[derive(Clone, Copy)] Leptos context struct carrying all per-block reactive UI state (id, expanded, show_reprompt, reprompt_value). BlockRenderer provides it via a single provide_context(block_ctx). BrowseBar drops its block_id and on_expand props and reads from context. generic.rs ExternalUrl arm migrated to BlockContext.

Test plan

• cargo test -p papillon-shared — 250 tests pass
• cargo check -p papillon — zero warnings (unused CanvasBlock import removed)
• cargo check --target wasm32-unknown-unknown (frontend) — clean
• grep -rn "SourceBlockId" apps/papillon/ — zero source matches
• grep -n "linked_block_ids.*Vec::new\|auto_expand.*false" commands/canvas.rs — zero matches
• Manual smoke: submit prompt → phase dots animate, prompt_text preserved on final block
• Manual smoke: browse block auto-expand retained across all phase events
• Manual smoke: in-block URL bar navigates to linked block, old block collapses

🤖 Generated with Claude Code

[github-actions]

Benchmark Regression Report

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

  ed25519_keypair_generation                17.3 µs  (baseline: 19.7 µs, -12.5%)  [ok]
  did_key_derivation                         1.4 µs  (baseline: 1.5 µs, -7.9%)  [ok]
  mandate_create_sign                       21.9 µs  (baseline: 23.9 µs, -8.5%)  [ok]
  mandate_chain_verify_depth3              139.2 µs  (baseline: 126.9 µs, +9.6%)  [ok]
  sd_jwt_issue_5claims                      25.1 µs  (baseline: 27.9 µs, -9.7%)  [ok]
  sd_jwt_verify_disclose_3of5               49.0 µs  (baseline: 44.3 µs, +10.6%)  [ok]
  session_open_full_lifecycle               98.0 µs  (baseline: 108.7 µs, -9.8%)  [ok]
  receipt_create_cosign                     44.2 µs  (baseline: 48.9 µs, -9.7%)  [ok]
  federation_announce_local                 53.7 µs  (baseline: 56.0 µs, -4.1%)  [ok]

All benchmarks within 30% of baseline.

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