feat(standalone): floating sheet navigator panel backed by a C++ hierarchy export (sheet-panel)

Canvas-only eeschema sessions (read-only viewers, hide-UI editors) could not
reach the wx hierarchy pane, so a viewer of a hierarchical design was stuck
on the entry sheet. Adds the React stand-in (SheetPanel) + the bridge:

- eeschema_embind: kicadSheetsGetTree() (SCHEMATIC::Hierarchy(), page-number
  ordered: instance KIID path / parent / Sheetname / file / page / depth) and
  kicadSheetsEnter(path) (validated synchronously, applied on the coroutine
  via SCH_ACTIONS::changeSheet — same route as the wx pane, history intact).
  OnSchSheetChanged pushes window.kicadCollab.onSheetsState after EVERY
  navigation, wx-driven included, so the panel is event-driven.
- kicad → 9ab93b838e: navigate actions allowlisted for read-only viewers.
- SheetPanel.tsx: LayerPanel shell conventions; rendered + menu row only when
  the hierarchy has >1 sheet instance; viewers boot it collapsed.
- tests: fixtures/demo/hier (root → Power, IO → Sub) + web/sheet-panel.spec
  (list/order/depth, navigate + collab rebind, menu toggle/close, flat
  schematic has no panel, bridge-driven navigation updates the panel);
  SheetPanel.test.ts for the wire parser.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CtN6ASBvMGNbjPqY5boycg

tests/fixtures/demo/hier/hier.kicad_sch

@@ -0,0 +1,49 @@
+(kicad_sch
+	(version 20250114)
+	(generator "eeschema")
+	(generator_version "9.0")
+	(uuid "a0000000-0000-4000-8000-000000000001")
+	(paper "A4")
+	(title_block
+		(title "Hierarchy demo — root")
+	)
+	(lib_symbols)
+	(wire
+		(pts (xy 50.8 101.6) (xy 101.6 101.6))
+		(stroke (width 0) (type default))
+		(uuid "c0000000-0000-4000-8000-000000000011")
+	)
+	(sheet
+		(at 50.8 50.8)
+		(size 25.4 20.32)
+		(exclude_from_sim no)
+		(in_bom yes)
+		(on_board yes)
+		(dnp no)
+		(fields_autoplaced yes)
+		(stroke (width 0.1524) (type solid))
+		(fill (color 0 0 0 0.0000))
+		(uuid "a0000000-0000-4000-8000-0000000000a1")
+		(property "Sheetname" "Power" (at 50.8 50.0876 0) (effects (font (size 1.27 1.27)) (justify left bottom)))
+		(property "Sheetfile" "power.kicad_sch" (at 50.8 71.7046 0) (effects (font (size 1.27 1.27)) (justify left top)))
+		(instances (project "hier" (path "/a0000000-0000-4000-8000-000000000001" (page "2"))))
+	)
+	(sheet
+		(at 101.6 50.8)
+		(size 25.4 20.32)
+		(exclude_from_sim no)
+		(in_bom yes)
+		(on_board yes)
+		(dnp no)
+		(fields_autoplaced yes)
+		(stroke (width 0.1524) (type solid))
+		(fill (color 0 0 0 0.0000))
+		(uuid "a0000000-0000-4000-8000-0000000000a2")
+		(property "Sheetname" "IO" (at 101.6 50.0876 0) (effects (font (size 1.27 1.27)) (justify left bottom)))
+		(property "Sheetfile" "io.kicad_sch" (at 101.6 71.7046 0) (effects (font (size 1.27 1.27)) (justify left top)))
+		(instances (project "hier" (path "/a0000000-0000-4000-8000-000000000001" (page "3"))))
+	)
+	(sheet_instances
+		(path "/" (page "1"))
+	)
+)

tests/fixtures/demo/hier/io.kicad_sch

@@ -0,0 +1,31 @@
+(kicad_sch
+	(version 20250114)
+	(generator "eeschema")
+	(generator_version "9.0")
+	(uuid "b0000000-0000-4000-8000-000000000003")
+	(paper "A4")
+	(title_block
+		(title "Hierarchy demo — io")
+	)
+	(lib_symbols)
+	(wire
+		(pts (xy 50.8 101.6) (xy 101.6 101.6))
+		(stroke (width 0) (type default))
+		(uuid "c0000000-0000-4000-8000-000000000013")
+	)
+	(sheet
+		(at 50.8 50.8)
+		(size 25.4 20.32)
+		(exclude_from_sim no)
+		(in_bom yes)
+		(on_board yes)
+		(dnp no)
+		(fields_autoplaced yes)
+		(stroke (width 0.1524) (type solid))
+		(fill (color 0 0 0 0.0000))
+		(uuid "a0000000-0000-4000-8000-0000000000a3")
+		(property "Sheetname" "Sub" (at 50.8 50.0876 0) (effects (font (size 1.27 1.27)) (justify left bottom)))
+		(property "Sheetfile" "io_sub.kicad_sch" (at 50.8 71.7046 0) (effects (font (size 1.27 1.27)) (justify left top)))
+		(instances (project "hier" (path "/a0000000-0000-4000-8000-000000000001/a0000000-0000-4000-8000-0000000000a2" (page "4"))))
+	)
+)

tests/fixtures/demo/hier/io_sub.kicad_sch

@@ -0,0 +1,16 @@
+(kicad_sch
+	(version 20250114)
+	(generator "eeschema")
+	(generator_version "9.0")
+	(uuid "b0000000-0000-4000-8000-000000000004")
+	(paper "A4")
+	(title_block
+		(title "Hierarchy demo — io sub")
+	)
+	(lib_symbols)
+	(wire
+		(pts (xy 50.8 101.6) (xy 101.6 101.6))
+		(stroke (width 0) (type default))
+		(uuid "c0000000-0000-4000-8000-000000000014")
+	)
+)

tests/fixtures/demo/hier/power.kicad_sch

@@ -0,0 +1,16 @@
+(kicad_sch
+	(version 20250114)
+	(generator "eeschema")
+	(generator_version "9.0")
+	(uuid "b0000000-0000-4000-8000-000000000002")
+	(paper "A4")
+	(title_block
+		(title "Hierarchy demo — power")
+	)
+	(lib_symbols)
+	(wire
+		(pts (xy 50.8 101.6) (xy 101.6 101.6))
+		(stroke (width 0) (type default))
+		(uuid "c0000000-0000-4000-8000-000000000012")
+	)
+)