feat: backend mirror of the junction limits (#329 §5, AC9)

custom_components/houseplan/junction_limits.py repeats П1-П4 for the write
barrier in websocket_api, counting per rule so an inherited violation still
round-trips, and raises JunctionLimitError with the stable code
junction_limit_<rule>.

П5 is deliberately not mirrored — it judges the rendered wall bodies, and a
second mitre/inset pipeline in Python would drift more dangerously than the
rule it guards. Optimize stays outside the check for the same reason migration
and import do: it repairs existing geometry.

test_parity_with_the_frontend_checks feeds identical fixtures to the TS
functions and to this module and demands the same verdict, so the two
implementations cannot silently diverge.

Issue: #329
User-Visible: no

test/golden-matrix.test.mjs

@@ -367,7 +367,7 @@ test('sun-ray golden requires browser-painted light from a state-only sun entity
   assert.ok(scenario);
   const fixture = prepareGoldenFixture(scenario);
   const space = fixture.config.spaces.find((item) => item.id === scenario.space);
-  assert.equal(GOLDEN_MATRIX_VERSION, 47);
+  assert.equal(GOLDEN_MATRIX_VERSION, 48);
   assert.equal(space.settings.sun_rays, true);
   assert.equal(scenario.northDeg, 90,
     'the sign-sensitive golden must keep a non-zero north direction');