docs: update KB and project docs with 2026-02-11 progress

- KB Gen 003: NX version (DesigncenterNX 2512), first real results
- sol101-static.md: path resolution lessons, in-place solving, result extraction confirmed
- CONTEXT.md: solver pipeline operational, first results (disp=17.93mm, stress=111.9MPa)
- DECISIONS.md: DEC-HB-008 to DEC-HB-011 (backup/restore, iteration arch, history DB, git workflow)
- optimization_engine/README.md: created (DesigncenterNX support, path resolution, NX file refs)
- studies/01_doe_landscape/README.md: updated architecture, iteration folders, history DB
- _index.md: closed gaps G3,G4,G6,G10-G14, updated generation to 003

projects/hydrotech-beam/DECISIONS.md

@@ -52,3 +52,36 @@ Numbered decision log. Check here before proposing anything that contradicts a p
 - **Decision:** All project data lives in `/repos/Atomizer/projects/`. This is the single source of truth for all current and future projects. Synced to Windows (dalidou) via Syncthing. No duplicate local folders.
 - **Rationale:** One place for everything — agents, CEO, and Windows all reference the same files. Version-controlled via Gitea, bidirectional sync via Syncthing.
 - **Status:** Approved
+
+## DEC-HB-008: Backup/restore in-place solving (not iteration copies)
+- **Date:** 2026-02-11
+- **By:** CEO + Technical Lead 🔧
+- **Decision:** Solve on master model files **in-place** (in `models/` directory) using backup/restore for iteration isolation. Do NOT copy `.sim`/`.fem`/`.prt` files to per-iteration folders.
+- **Rationale:** NX `.sim` files store absolute internal references to `.fem` and `.prt` files. Copying model files to iteration folders breaks these references (`Parts.Open` returns `None`). Backup/restore preserves all internal references while still providing isolation between trials.
+- **Workflow per trial:**
+  1. Restore master model files from backup
+  2. Write `.exp` file with trial DVs, open `.sim`, rebuild, solve
+  3. Archive outputs (OP2, F06, params.json, results.json) to `iterations/iterNNN/`
+  4. Iteration folders contain outputs only — NOT model files
+- **Status:** Approved — implemented and tested
+
+## DEC-HB-009: Iteration folder architecture
+- **Date:** 2026-02-11
+- **By:** CEO + Technical Lead 🔧
+- **Decision:** Each trial archived to `studies/01_doe_landscape/iterations/iterNNN/` with: `params.json`, `params.exp`, `results.json`, OP2, F06.
+- **Rationale:** Full traceability per trial. Any result can be audited, re-examined, or debugged. Smart retention policy keeps last 10 + best 3 with full data, strips the rest to save space.
+- **Status:** Approved — implemented in `iteration_manager.py`
+
+## DEC-HB-010: Persistent history database
+- **Date:** 2026-02-11
+- **By:** Technical Lead 🔧
+- **Decision:** Maintain `history.db` (SQLite, append-only) + `history.csv` that survives `--clean` and Optuna resets. Logs all DVs, results, feasibility, timestamps across all studies.
+- **Rationale:** Optuna DB is study-scoped and can be cleaned/reset. History DB provides permanent record across studies, enabling cross-study learning, debugging, and auditability. Append-only means no data loss.
+- **Status:** Approved — implemented and tested
+
+## DEC-HB-011: Git-only development workflow (Syncthing paused)
+- **Date:** 2026-02-11
+- **By:** CEO
+- **Decision:** During active development, use Git (push/pull) exclusively for code sync between server and dalidou. Syncthing paused to avoid conflicts.
+- **Rationale:** Syncthing's bidirectional sync creates conflicts when both sides edit files simultaneously during development. Git provides explicit merge control.
+- **Status:** Active — resume Syncthing for production/delivery phases