docs(mcp): document plan↔world coordinate convention (#356)

Adds a Coordinate conventions section to packages/mcp/README.md and a 71-node MCP demo scene that exercises every claim it makes. Closes #337.

Covers: right-handed scene with X/Z ground plane and Y up; metres for lengths and radians for rotations as Euler [x, y, z]; the [x, z] → (x, y, z) plan-to-world mapping with no sign flip in the stored contract; level/building-local framing (world only under identity transform); rotation-not-reflection caveat for the 2-D plan panel and the iso-default top-down azimuth offset; a worked 30° rotated slab example; and the wall-local-metres trap for door/window/place_item coordinates.

Companion: examples/coordinate-conventions-demo.{json,md} with a reference compass at the origin and Demos A/B/C/D illustrating axis-aligned baseline, the rotated example, and the page-intent vs world-result L pair.

Co-authored-by: Marcel Gruber <marcel@grubertech.com>

packages/mcp/examples/coordinate-conventions-demo.md

@@ -0,0 +1,68 @@
+# Coordinate-conventions demo
+
+Companion scene for the **Coordinate conventions** section of
+[`../README.md`](../README.md). Stress-tests every claim that section
+makes against real Pascal-generated geometry, built entirely through the
+MCP API.
+
+## Load it
+
+```bash
+pascal-mcp --stdio --scene examples/coordinate-conventions-demo.json
+```
+
+The scene fits in a 50 m × 50 m ground-plane footprint at `level: 0`.
+
+## What's in the scene
+
+A flat ground level with four demos, plus a labelled compass at the
+origin so the world axes are unambiguous regardless of which viewport
+you're in.
+
+| Section | What | Why |
+|---|---|---|
+| Reference compass at the origin | 12 m `+X` and `+Z` axis bars with arrowhead tips, short `-X` / `-Z` stubs, an origin marker, plus zone labels naming each component of `[x, z]`. | Establishes which way `+X` and `+Z` actually point in world space, independent of viewport rotation. |
+| **Demo A** at world `(18, 0)` | Axis-aligned 6 m × 4 m rectangle. | Baseline that matches the only example currently in `examples/generate-apartment.md` style (`[0,0] → [10,0]`). |
+| **Demo B** at world `(18, 10)` | The proposed README example, **verbatim**: `polygon: [[0,0],[5.196,3.0],[3.196,6.464],[-2.0,3.464]]` at this offset. | Programmatically verified to be a 6 m × 4 m rectangle whose first edge is heading 30° CCW from `+X` (side lengths 6/4/6/4 m to 4 dp, `AB · AD = 0`, heading = 30.0007°). Confirms the README's worked example produces the geometry it claims. |
+| **Demo C** at world `(0, 22)` and `(10, 22)` | An **L** authored on a north-up page (page-+x right, page-+y up) drawn at half scale and faded; alongside it, the same L pasted into `[x, z]` **uncorrected**, drawn at full scale and vivid. | Makes the external-coordinate gotcha visual: the author's page-+y direction lands on world +Z, which does not correspond to "screen-up" in any of Pascal's viewports. |
+| **Demo D** at world `(22, 22)` and `(32, 22)` | Same pairing for the L with its second coordinate reflected (`z → 5 − z`) before paste. | Makes plain that z-reflection only "corrects" a true mirror; Pascal's viewports apply a *rotation*, so the reflection produces a mirror-image of Demo C rather than a page-correct L. |
+| Takeaway band on the south edge | Short labels summarising the convention. | Self-documenting; readable in any viewport without an external README. |
+
+## What it confirms
+
+Open the scene in Pascal (or render it from the JSON) and you can read
+each claim directly off the geometry:
+
+1. **`[x, z] → world (x, 0, z)` is exact, no sign flip.** Demo A and
+   Demo B both sit flat on the floor at Y = 0; their polygon vertices
+   round-trip through `save_scene` / `get_scene` byte-identical to what
+   was authored (graph hash matches, see `validate_scene` output).
+
+2. **Demo B is the rectangle the README says it is.** A simple analytic
+   check on its 4 vertices yields side lengths 6, 4, 6, 4 m (to 4 dp)
+   with the first edge at 30.0007° from +X and perpendicular adjacent
+   edges. No rendering required.
+
+3. **External page coordinates *rotate* (not mirror) when pasted as
+   `[x, z]` and viewed in Pascal.** Inspecting Demo C in the 2-D plan
+   panel: the page-up L lands rotated 90° clockwise, so the author's
+   "stem-up, foot-bottom-right" reads as "stem-on-right, foot-along-top"
+   on screen. Inspecting Demo D right next to it: the z-reflected
+   variant lands as the *mirror* of Demo C — neither matches a
+   page-correct L. That demonstrates why "reflect across the axis" is
+   the wrong corrective for an issue that is, in this viewport, a
+   rotation.
+
+4. **The 3-D "top-down" snap is offset 45° from world axes by default.**
+   Viewing the scene in 3-D and snapping to top-down from the default
+   iso camera shows the rectangular site polygon as a *diamond* (its
+   long edges are diagonals on screen, not axis-aligned). That's the
+   camera's "up" vector being inherited from the iso start; once you
+   orbit to a true axis-aligned top-down it goes away. The label
+   inviting reviewers to "verify against a guide image" exists for this
+   reason.
+
+## Reproducibility
+
+The canonical demo is the JSON file in this directory; load that file to compare
+the geometry against the notes above.