docs: design for open spans and wall-centric Delete

Capture brainstorming decisions for partial virtual walls and Delete merge/room flow before implementation.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/superpowers/specs/2026-08-05-open-spans-delete-design.md

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+# Open spans + wall-centric Delete — design
+
+Status: **approved in chat** (2026-08-05). Implementation plan follows after
+spec review.
+
+## Goal
+
+Replace whole-boundary open-wall toggle with **partial virtual stretches**
+(anchor → second point on a shared wall), and make **Delete** operate on walls
+(close virtual → merge / delete room) with confirmation.
+
+## Current behaviour (baseline)
+
+- Tool «Открытая граница»: one click toggles `room.open_to` for a room pair;
+  **all** `sharedBoundary` stretches between the pair become virtual.
+- Tool «Удалить»: click inside a room → confirm → delete room.
+- No geometric open-span storage; light zones use `open_to` only.
+
+## Decisions (from brainstorming)
+
+| Topic | Decision |
+|-------|----------|
+| Data model | Approach **1**: `space.open_spans` + keep `open_to` as connectivity index |
+| Open target | Shared wall only; outer wall → toast |
+| Gesture | Click1 = anchor P1; click2 = P2 on same edge; P2 clamped to nearest corners either side of P1 |
+| Cursor (openwall) | **crosshair** (same as Draw), not pointer |
+| Snap | Corner / existing open joint first; else grid projected onto the wall |
+| Close virtual | Both openwall (click on span) and Delete (first hit on span) |
+| Thickness on open | Clear `walls[]` cm on covered keys immediately |
+| Thickness on close | Inherit cm from remaining solid collinear part of same wall; if none → `DRAW_WALL_DEFAULT_CM` (15 cm / imperial) |
+| Delete + virtual | First click closes span (no confirm) |
+| Delete + shared solid | Confirm → **merge** the two rooms along the **entire** shared boundary |
+| Delete + outer wall | Confirm → **delete room** |
+| Delete + inside room | Same as outer: confirm → delete room |
+| Merge / delete room | Always `confirm()` |
+| Openings on virtual | **Forbidden**; remove any openings on a span when it becomes virtual; cannot place new ones there |
+| Legacy `open_to` | On read, treat full `sharedBoundary` of the pair as open spans; persist `open_spans` on first save/edit |
+
+---
+
+## §1 Data model
+
+### Storage (per space)
+
+- `open_spans: Array<{ key, … }>` — geometric virtual stretches. Keying aligns
+  with `walls[]` (quantised midpoint + direction mod 180°). Store enough to
+  recover segment endpoints after rekey (explicit `a`/`b` normalised coords
+  and/or length along the edge — implementation detail in the plan).
+- `rooms[].open_to: string[]` — symmetric room-id links for
+  `openZoneOf` / glow (unchanged consumer API).
+
+### Invariants
+
+1. A span exists only on a **shared** boundary of exactly two rooms.
+2. While a pair has ≥1 span → both rooms list each other in `open_to`.
+   Removing the last span clears the link.
+3. An open span and a positive `walls[]` thickness on the same covered keys
+   do not coexist: opening clears thickness; closing restores per rules above.
+4. Legacy: `open_to` present, `open_spans` absent/empty → **read path** expands
+   each linked pair to its full `sharedBoundary` segment list as virtual spans;
+   write them into `open_spans` on the next config save that touches the space
+   (or any openwall/delete edit).
+5. Geometry lifecycle: resize / undo / scale / split / merge **rekey** spans
+   like walls; unmatched keys degrade silently; merge of a pair drops spans
+   between those two rooms.
+
+---
+
+## §2 Gestures
+
+### Tool: Open boundary (`openwall`)
+
+1. **Hover** on a shared solid wall: highlight eligible wall; cursor
+   **crosshair**. Outer wall: no “hot” affordance beyond default crosshair;
+   click → toast (shared only).
+2. **Click1**: anchor P1 on the wall (snap: corner/joint → else grid on edge).
+   Remember the edge and nearest corners L, R along that edge.
+3. **Preview**: segment P1→cursor, clamped to [L, R] on the same edge.
+4. **Click2**: P2 in [L, R]. If length below minimum → toast, clear anchor.
+   Else write span, clear thickness on covered keys, remove openings on the
+   span, refresh `open_to`.
+5. **Click on existing virtual span**: close that span; restore thickness
+   (neighbour solid / default 15); no confirm.
+6. **Esc**, click away, tool change, leave markup: clear in-progress anchor.
+
+### Tool: Delete (`delroom` — wall-centric)
+
+Hit priority: virtual span → shared solid wall → outer wall → point inside room.
+
+| Hit | Action |
+|-----|--------|
+| Virtual span | Close span (no confirm), same as openwall |
+| Shared solid wall | Confirm → merge both rooms (entire shared boundary) |
+| Outer wall | Confirm → delete the room that owns that outer edge |
+| Inside room | Confirm → delete that room |
+| Miss | No-op (optional light toast) |
+
+After closing a virtual span, a later Delete click on that now-solid shared
+wall runs the merge path (with confirm).
+
+---
+
+## §3 Thickness, openings, errors, tests
+
+### Thickness
+
+- Opening a span: strip `walls[]` entries (or null cm) for keys covered by the
+  span (atomic collinear pieces under the segment).
+- Closing a span: if a collinear solid remainder of the same wall still has cm,
+  copy that cm onto the newly solid stretch; else apply `DRAW_WALL_DEFAULT_CM`.
+- Wall-thickness tool continues to refuse open stretches (toast).
+
+### Openings (doors / windows)
+
+- **Not allowed** on virtual spans.
+- When a stretch becomes virtual: **delete** any openings whose centre lies on
+  that stretch (same association rules as wall bodies / angle-aware hit).
+- Opening tool: refuse placement on a virtual span (toast).
+
+### Errors / cancel
+
+- Outer / not-a-wall / too short / click2 off the anchored edge → toast and
+  clear the in-progress anchor.
+- Merge and room delete always go through `confirm()`; cancel → no-op.
+- Saves stay optimistic like other plan edits.
+
+### Testing
+
+**Unit**
+
+- Clamp P2 to nearest corners; min length.
+- Span write ↔ `open_to` add/remove.
+- Thickness clear on open; restore from neighbour / default 15 on close.
+- Legacy `open_to` → virtual full sharedBoundary spans.
+- Rekey after edge move / scale; degrade orphans.
+- Opening removal when span opens; placement blocked on virtual.
+
+**Smoke**
+
+- Anchor + P2 → dashed span; glow crosses only when span links rooms.
+- Close via openwall and via Delete.
+- Delete merge / delete-room confirms; outer open attempt toasts.
+- Crosshair cursor in openwall tool.
+
+### Docs / i18n
+
+- Update `docs/ARCHITECTURE.md` (open boundaries), `WALL-THICKNESS.md` (open
+  refuses thickness + clear on open), `UX-MODES` / TESTING hints, CHANGELOG.
+- New toasts: shared-only, short segment, openings removed, delete/merge
+  confirms copy; refresh `title.markup_openwall` / `delroom` hints.
+
+### Out of scope
+
+- Partial merge along only the former virtual segment (explicitly rejected: merge is whole shared boundary).
+- Outer-wall virtual openings (“to outdoors”).
+- Restoring thickness from pre-open snapshot (neighbour / default only).
+- Changing glow math beyond “any span ⇒ rooms linked”.
+
+---
+
+## Implementation sketch (not a plan)
+
+1. Pure helpers: expand legacy, clamp segment, span CRUD, thickness
+   clear/restore, opening purge on span, rekey/degrade.
+2. Card: openwall two-click state machine; Delete hit priority + confirms.
+3. Render: dashed strokes from `open_spans` (fallback legacy expand); trim
+   outlines/cuts from span list.
+4. Smokes + unit tests; docs/i18n; version bump when shipping.
+
+## Open points for the implementation plan only
+
+- Exact JSON shape of one `open_spans` entry (`key` only vs `key`+`a`+`b`).
+- Minimum segment length (e.g. `gridPitch` or fixed cm).
+- Whether Delete merge reuses the existing merge-tool code path verbatim.