docs: add v2 architecture planning and research findings

Architecture decision: Tauri 2.x + Svelte 5 + Claude Agent SDK (Node.js sidecar).
Includes research on Agent SDK streaming, xterm.js performance, WebKit2GTK
limitations, and adversarial review corrections (two-tier observation, Svelte 5
over Solid.js, SDK abstraction layer). Six implementation phases defined,
MVP = Phases 1-4.

docs/progress.md

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+# BTerminal v2 — Progress Log
+
+## Session: 2026-03-05
+
+### Research Phase (complete)
+- [x] Analyzed current BTerminal v1 codebase (2092 lines Python, GTK3+VTE)
+- [x] Queried Memora — no existing BTerminal memories
+- [x] Researched Claude Agent SDK — found structured streaming, subagent tracking, hooks
+- [x] Researched Tauri + xterm.js ecosystem — found 4+ working projects
+- [x] Researched terminal latency benchmarks — xterm.js acceptable for AI output
+- [x] Researched 32:9 ultrawide layout patterns
+- [x] Evaluated GTK4 vs Tauri vs pure Rust — Tauri wins for this use case
+- [x] Created task_plan.md with 8 phases
+- [x] Created findings.md with 7 research areas
+
+### Technology Decision (complete)
+- Decision: **Tauri 2.x + Solid.js + Claude Agent SDK + xterm.js**
+- Rationale documented in task_plan.md Phase 0
+
+### Adversarial Review (complete)
+- [x] Spawned devil's advocate agent to attack the plan
+- [x] Identified 5 fatal/critical issues:
+  1. Node.js sidecar requirement unacknowledged
+  2. SDK 0.2.x instability — need abstraction layer
+  3. Three-tier observation overengineered → simplified to two-tier
+  4. Solid.js ecosystem too small → switched to Svelte 5
+  5. Missing: packaging, error handling, testing, responsive design
+- [x] Revised plan (Rev 2) incorporating all corrections
+- [x] Added error handling strategy table
+- [x] Added testing strategy table
+- [x] Defined MVP boundary (Phases 1-4)
+- [x] Added responsive layout requirement (1920px degraded mode)
+
+### Next Steps
+- [ ] Present plan to user for review and decision
+- [ ] Create feature branch
+- [ ] Begin Phase 1: Project scaffolding