docs: add design plans for API and auto-fill features

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/plans/2026-02-17-autofill-from-export-history-design.md

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+# Auto-fill Equipment/Drawing from Export History
+
+## Problem
+
+When a SolidWorks file is opened that doesn't match the `DrawingInfo` regex (e.g., `Conveyor Frame.sldasm` instead of `5028 A02 Conveyor.slddrw`), the equipment and drawing number dropdowns are left empty even though the file may have been exported before with known values.
+
+## Decision
+
+- **Lookup key:** SolidWorks source file path (`ActiveDocument.FilePath`)
+- **Storage:** Query the existing `ExportRecords` table (no new table or migration)
+- **Priority:** Database lookup first; fall back to title regex parse if no history found
+
+## Design
+
+### Modify `UpdateActiveDocumentDisplay()` in `MainForm.cs`
+
+When the active document changes:
+
+1. Query the DB for the most recent `ExportRecord` where `SourceFilePath` matches `activeDoc.FilePath` (case-insensitive)
+2. If found, parse the stored `DrawingNumber` via `DrawingInfo.Parse()` and auto-fill equipment/drawing dropdowns
+3. If not found, fall back to current behavior: `DrawingInfo.Parse(activeDoc.Title)`
+
+### What doesn't change
+
+- No schema changes, no new migration
+- Equipment/drawing dropdowns still populated with historical values in `InitializeDrawingDropdowns()`
+- Export flow untouched
+
+### Error handling
+
+- DB query wrapped in try/catch so failures don't break the UI
+- Case-insensitive path comparison (Windows paths are case-insensitive)