docs: add release notes for v0.4.0

docs/release-notes/v0.4.0.md

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+# v0.4.0 — Performance & stability overhaul
+
+## ✨ New features
+
+**Terminal performance overhaul**
+* Heavy output (long boot logs, big file dumps) now renders much more smoothly — drawing was
+  reworked to stop re-creating fonts and brushes on every frame.
+* Scroll-back history storage was rebuilt so the terminal stays fast no matter how much output
+  has accumulated; previously it got slower and slower once the history buffer filled up.
+
+**Full-screen apps (vim / htop) behave like a real terminal**
+* The mouse wheel now scrolls inside full-screen tools like `vim` and `htop` instead of doing nothing.
+* Tools that ask the terminal "where is the cursor?" now get an answer — fixes full-screen apps
+  that could hang on startup or draw at the wrong position.
+
+**Connection status you can actually see**
+* When an SSH connection drops or hits an error, a message now appears right in the tab —
+  no more sessions that silently stop responding.
+* When the local shell exits (e.g. you type `exit`), the tab shows
+  `[ETTerms] shell process exited` instead of just going dead.
+
+**Faster PDU status (GUI and AI)**
+* Outlet polling now bundles all 12 ports into a few SNMP requests instead of 36 separate ones —
+  the Status page refreshes faster, and an unreachable PDU no longer stalls the poll for minutes.
+* The AI `pdu_status` tool gets the same speed-up.
+
+## 🐛 Bug fixes
+
+**Garbled Chinese / non-English text**
+* Fixed: multi-byte characters could occasionally turn into `�` garbage when text was split
+  across data chunks — this affected TTL script `wait` matching, what the AI reads over the
+  serial bridge, and saved session log files. All three now decode correctly.
+
+**Scrolling**
+* Fixed: while you were scrolled back reading history, any new output yanked the view down to
+  the bottom. The view now stays where you are and only follows when you're already at the bottom.
+
+**Resource & stability**
+* Fixed small memory and handle leaks when opening and closing many local shell tabs over a
+  long session.
+* Fixed: typing into an SSH session after the connection died could crash the window — it now
+  just prints a short "write failed" note in the tab.
+
+## 📦 Downloads
+
+Two builds are produced:
+
+| Build | Needs .NET 8 Desktop Runtime? | Notes |
+|-------|-------------------------------|-------|
+| **Standard** (`ETTerms_v0.4.0`) | ✅ Yes | Smaller; for machines that already have the runtime |
+| **Portable** (`ETTerms_v0.4.0_portable`) | ❌ No | Runtime bundled — unzip and run, no install / admin needed |
+
+Run `ETTerms v0.4.0.exe`.
+
+## 🔗 Links
+* PDU MCP & Serial MCP setup guide:
+  [docs/serial-mcp-guide.md](https://github.com/ETWen/ETTerms/blob/main/docs/serial-mcp-guide.md)
+* TTL reference:
+  [docs/ttl-script-reference.md](https://github.com/ETWen/ETTerms/blob/main/docs/ttl-script-reference.md)
+
+**Full changelog:** [v0.3.3...v0.4.0](https://github.com/ETWen/ETTerms/compare/v0.3.3...v0.4.0)