feat(task-5): sections, cut faces, material hatch and detail views

engineering/mech/views.py grows the two remaining view kinds. A section's cut
faces come out of the same radial profile the silhouette does, so the hatched
area and the drawn outline are one calculation and cannot disagree: a solid
shaft's half-section loop measures exactly the sum of its segment rectangles,
and a bore closes the loop on itself rather than being hatched over.

ISO 6410's section convention falls out of the model rather than being special-
cased: an internal thread's radial_edit already raises the bore to the major
diameter, so the hatch stops there. ISO 128-3's rib rule is a flag on the
feature - a pocket marked no_section_hatch is drawn and left unhatched, while
the rest of the cut face is hatched with the part's ISO 128-50 material.

Four styles, each either implemented or refused by name with the alternative
rather than approximated: full and half on a longitudinal plane through a
revolved axis; full and revolved on a transverse plane (the annulus, flattened
at 2 degrees - 0.015% of the radius, stated in CIRCLE_SEGMENTS rather than
hidden); full and offset on a prismatic part, where through features split the
cut face and pockets reduce it to the remaining thickness. A plane that misses
the part, or that grazes an outline vertex so its crossings cannot be paired, is
refused instead of producing a guessed cut face.

Details scale a circular region of a parent view, emit the ISO 128-34 circle and
letter for the parent and the "A (5:1)" label for the detail itself, and report
every feature that falls outside the circle in omitted - the honesty rule again.

Three test assertions were corrected to the measured truth rather than the
engine bent to them, each verified by running the code:

* the ISO 6410 test asked for the whole cut-face loop's minimum to be the M20
  major (r=10). It is not, and must not be: over the 5 mm that is drilled but
  not threaded the loop closes on the real 16 mm drill (r=8). The loop is
  ((0,10),(0,25),(25,25),(30,25),(30,8),(30,8),(25,8),(25,10),(0,10)). The test
  now pins both spans, which is the convention it meant to assert.
* the transverse-annulus areas were pinned at rel=2e-4. An inscribed regular
  180-gon is exactly 1 - (n/2pi) sin(2pi/n) = 2.0307e-4 under the circle, so
  2e-4 rejects the very flattening CIRCLE_SEGMENTS declares. The tolerance is
  now 2.1e-4 and the test computes the deficit and pins it, so the number is
  derived rather than guessed. CIRCLE_SEGMENTS' docstring now states both
  figures: 0.015% radial (the 2 degree chord's sagitta) and 0.0203% of area.
* tests/test_mech_views.py pinned VIEW_KINDS == ("front","side","top"), which
  this task deliberately grows. It now pins the three orthographic kinds as the
  first three and the full five-kind tuple.

_is_longitudinal takes only the plane; the plan's signature also took the part
and never read it.

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

engineering/mech/__init__.py

@@ -68,14 +68,22 @@ from .primitives import (
     translate,
 )
 from .views import (
+    CIRCLE_SEGMENTS,
     PROJECTIONS,
+    SECTION_PLANE,
+    SECTION_STYLES,
     VIEW_KINDS,
     Band,
     View,
     build_view,
+    cut_loops,
+    detail_marker_prims,
+    detail_view,
     layout_origin,
+    normalise_plane,
     outline_prims,
     radial_profile,
+    section_view,
     silhouette_prims,
 )
 
@@ -84,6 +92,7 @@ __all__ = [
     "AxialBore",
     "Band",
     "BendLine",
+    "CIRCLE_SEGMENTS",
     "CentreHole",
     "Chamfer",
     "Circle",
@@ -114,6 +123,8 @@ __all__ = [
     "RetainingGroove",
     "RevolvedPart",
     "Role",
+    "SECTION_PLANE",
+    "SECTION_STYLES",
     "Segment",
     "Slot",
     "Text",
@@ -124,11 +135,15 @@ __all__ = [
     "bbox",
     "build_part",
     "build_view",
+    "cut_loops",
+    "detail_marker_prims",
+    "detail_view",
     "feature_from_dict",
     "inner_radius_at",
     "layer_for",
     "layout_origin",
     "mirror_about_x",
+    "normalise_plane",
     "outer_radius_at",
     "outline_bbox",
     "outline_prims",
@@ -140,6 +155,7 @@ __all__ = [
     "radial_profile",
     "rotate",
     "scale",
+    "section_view",
     "segment_bounds",
     "silhouette_prims",
     "translate",