Shared drag placement; drags snap one block to join a diagonal (t67 phases 3-4)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/agent-log.md

@@ -4,6 +4,23 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-27: t67 phase 3 shared drag placement, and snapping (Claude Code)
+
+- `dragPlan(P, a, b, shift)` in `src/ui/selection.js` gives code, direction, variant, blocks and
+  a planned map for every block piece. `primaryRelease` and the drag preview both use it (the
+  duplicated `blockLine(...)` expression and the screw/slope-only planned map are gone).
+- Snapping: `joinSnap` in `src/sim/joins.js` moves a line one block up or down when its first
+  block would not join but would join seamlessly there with a diagonal piece involved, and
+  every block there is free. Fixes the owner's report that an uphill belt started beside a flat
+  belt on the same level did not join like screws do: it now snaps up to the seamless join. Also
+  covers t58's flat belt level with a downhill low end (snaps down) and a diagonal screw started
+  one block off. Straight beside straight never snaps. `joinIsDrop` (same-row feed into a slope's
+  low end) is now the one source for the drop, used by snapping and belt drawing.
+- Files: `src/sim/joins.js`, `src/ui/selection.js`, `src/ui/actions.js`, `src/ui/previews.js`,
+  `src/render/slope-belts.js`, `test/t_joins.js`, `test/t_slope_belts.js`.
+- Tests: `python3 test/run_all.py --browser` all passed; 9 snap checks plus a real
+  `primaryRelease` check. Open: phase 4's "doesn't connect" mark is not built yet.
+
 ## 2026-10-27: t66 dev console (Claude Code + Codex)
 
 - Backtick opens a dev console in any build. The first time in a world it warns that the save