feat(lite): add ObsidianDragonLite build mode and gate full-node features

Add --lite build flow and ObsidianDragonLite target naming, hide full-node pages/features in lite mode, enforce pool-only mining in lite, and include chat port feasibility audit documentation.

docs/chat-port-feasibility-2026-05-06.md

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+# Chat Port Feasibility Audit (2026-05-06)
+
+Scope:
+- Evaluate whether SilentDragonXLite chat can be ported to the current full-node ImGui wallet.
+- If feasible, define a safe path with feature flagging.
+
+## Source Audit (SilentDragonXLite)
+
+Primary chat surfaces in the reference codebase:
+- `external/SilentDragonXLite/src/chatmodel.h`
+- `external/SilentDragonXLite/src/chatmodel.cpp`
+- `external/SilentDragonXLite/src/Chat/Chat.h`
+- `external/SilentDragonXLite/src/Chat/Chat.cpp`
+- `external/SilentDragonXLite/src/controller.cpp` (transaction scan + memo decrypt/encrypt)
+- `external/SilentDragonXLite/src/Model/ChatItem.*`
+- `external/SilentDragonXLite/src/Model/ContactRequest*`
+- `external/SilentDragonXLite/src/DataStore/ChatDataStore.*`
+- `external/SilentDragonXLite/src/mainwindow.ui` and related Qt UI widgets.
+
+Observed characteristics:
+- Chat is tightly coupled to Qt widget/UI classes (`QListView`, `QStandardItemModel`, `.ui` forms).
+- Message transport relies on memo payload parsing/decryption embedded directly in transaction refresh/controller flow.
+- Contact requests, chat IDs, and message rendering are mixed with Qt view models and DataStore semantics.
+- Crypto flow depends on shared secret derivation and memo header conventions implemented inline in controller/chat code.
+
+## Current Wallet Architecture Gap
+
+Current app (`src/`) is Dear ImGui + SDL3 with different state/render model:
+- No existing chat domain model/service in ImGui code path.
+- No Qt runtime/view-model infrastructure to reuse reference chat modules.
+- Transaction refresh path is centralized in `NetworkRefreshService` and app refresh orchestration, not Qt controller callbacks.
+
+## Feasibility Decision
+
+Decision: **Not safe to directly port in this batch**.
+
+Reason:
+- A direct transplant would require substantial re-architecture, not incremental copy-over.
+- High risk of regressions in transaction refresh correctness, memo handling, and wallet UX stability.
+- Crypto/message parsing code requires dedicated security review before enabling in production.
+
+## Estimated Effort (for a proper staged port)
+
+1. Domain extraction (chat/contact/message models, storage interfaces): 3-5 days
+2. Service integration with current refresh pipeline (`NetworkRefreshService` + app): 4-7 days
+3. ImGui chat UI (list, composer, contact requests, actions): 4-6 days
+4. Security hardening + memo/crypto validation tests: 3-5 days
+5. End-to-end QA/regression/perf pass: 2-4 days
+
+Total estimate: **16-27 engineer-days**.
+
+## Risk Profile
+
+- Security risk: high (memo crypto path and malformed payload handling).
+- Stability risk: high (transaction refresh path is core-wallet critical).
+- UX risk: medium (new asynchronous chat states and contact flows).
+
+## Recommended Safe Plan
+
+1. Add compile/runtime feature flag (`DRAGONX_ENABLE_CHAT`) default OFF.
+2. Implement read-only parser + storage first (no send), fully test malformed memos.
+3. Add send path behind same flag with strict size/format guards.
+4. Ship disabled by default until security and regression acceptance criteria are met.