fix(task-12): restore the caller's tab from the layout listing; source the projection symbol

enter_layout read the caller's space from `backend._current_space`, which only
EzdxfBackend has. On ComBackend the getattr fell through to "Model", so
`sheet_frame` / `titleblock_apply` with a `layout=` dropped a live caller onto
the Model tab instead of putting them back -- the opposite of what the
docstring and both tool docs promise, and a dual-engine parity break in new
shared infrastructure. Measured on the live seat (AutoCAD 2026, scratch
document from drawing_new(), closed with discard):

    scratch document: Drawing24.dwg
    layouts: ['Layout1', 'Layout2', 'Model']
    B) current BEFORE: Layout2
    B) the value the OLD code read (backend._current_space):
       <<absent -> getattr default 'Model'>>

`layout_list()` already reported the right value two lines earlier on both
engines (ezdxf from $TILEMODE / the active layout, COM from
`doc.ActiveLayout.Name` -- read live as "Layout2" above), so `previous` now
comes from `listing["current"]`.

The projection-angle symbol was justified by an in-code derivation that cannot
decide the question, and the test restated the implementation. The derivation
was also applied inconsistently: the third-angle symbol mirrored the cone as
well as the end view, leaving the trapezoid's short side pointing AWAY from the
circles -- which is the first-angle arrangement. Transcribed instead from two
independent reproductions of the ISO 5456-2 figure, both downloaded and
measured (ISO's own figure is paywalled and was NOT read; the module says so):

  * FreeCAD's ISO 5457 sheet template A3_Landscape_ISO5457_advanced.svg,
    `first_angle_trapezoid` = "m 389,222 -10,2.5 v 5 l 10,2.5 z" -> 5-unit side
    at x=379, 10-unit side at x=389; circles r2.5/r5 at cx=396.
  * Wikimedia Commons "Convention placement vues dessin technique.svg", both
    symbols side by side, labelled FR and US: FR 35.856 @ x=13.343 / 66.135 @
    x=81.869, circles x=148.005; US 35.856 @ x=380.275 / 66.135 @ x=448.801,
    circles x=314.140.

Both draw the trapezoid the same way -- short side left -- and move only the
end view: first angle puts it right of the cone, third angle left. Equivalently
the short side points away from the circles in first angle and towards them in
third, which is how every source states it in words. `third` now follows.

Tests: the reference figures are transcribed into the test as data, and the
symbol is asserted to land its three landmarks in the order each figure does,
so the test cannot pass by agreeing with the code. The layout restore is pinned
through a proxy that hides `_current_space` from the real backend rather than a
fake that owns members ComBackend does not have. Both new tests fail on the
previous code (measured: "assert 16.0 < 8.0" for the mirrored cone; "- SheetB
+ Model" for the tab).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

tests/test_sheet_frames.py

@@ -266,3 +266,36 @@ async def test_a_missing_layout_is_refused_before_anything_is_drawn(backend):
     assert "NoSuchSheet" in str(excinfo.value)
     assert "layout_create" in str(excinfo.value)
     assert len(await backend.entity_list()) == before
+
+
+@pytest.mark.asyncio
+async def test_the_frame_puts_the_caller_back_on_the_tab_it_found_them_on(backend):
+    await backend.layout_create("SheetA")
+    await backend.layout_create("SheetB")
+    await backend.layout_set_current("SheetB")
+    await draw_sheet_frame(backend, "A3", zones=False, marks=False, layout="SheetA")
+    assert (await backend.layout_list())["current"] == "SheetB"
+
+
+@pytest.mark.asyncio
+async def test_the_caller_s_tab_is_read_from_the_listing_not_a_private_attribute(
+    backend, backend_without_private_state
+):
+    """`_current_space` exists on EzdxfBackend only. ComBackend keeps the live
+    tab in AutoCAD, so reading the private attribute there fell through to the
+    ``getattr`` default and dropped a live caller onto Model after every
+    border -- measured on AutoCAD 2026. `layout_list()["current"]` is the value
+    both engines really report, and `enter_layout` already had it in hand.
+    """
+    await backend.layout_create("SheetA")
+    await backend.layout_create("SheetB")
+    await backend.layout_set_current("SheetB")
+    with pytest.raises(AttributeError):
+        _ = backend_without_private_state._current_space
+
+    result = await draw_sheet_frame(
+        backend_without_private_state, "A3", zones=False, marks=False, layout="SheetA"
+    )
+
+    assert result["layout"] == "SheetA"
+    assert (await backend.layout_list())["current"] == "SheetB"