feat(task-3): the feature catalogue and its standards tables

engineering/mech/features.py is the nineteen typed features of spec section 5,
each answering the same four questions - is my placement legal, what do I
contribute to this view, to a cut face, and what dimension do I want - plus the
five structural hooks the view engine needs: removal_box, radial_edit,
corner_edit, outline_edit and omission. Pure arithmetic; a test asserts emitted
primitive tuples, not a screenshot.

Nothing restates a table the repository already owns: Keyway takes its width
and depth from engineering.keyway (DIN 6885-1) and is refused by name outside
that table's bore range; GearTeeth takes its outline from engineering.gear's
involute generator and is refused when its tip diameter disagrees with the
segment it sits on.

standards/threads.py transcribes ISO 261 Table 1 and Table 2 - first-choice
M1.6-M64 plus second-choice M14/M18/M22/M27/M33/M39/M45/M52/M60 - and computes
ISO 724's basic minor and pitch diameters from ISO 68-1's own formula, pinned
against the published values (M20x2.5 -> 17.294 / 18.376). The remaining
second-choice diameters, every third-choice diameter and fine pitches below M8
are not transcribed and are refused by name, never interpolated. The drawing
rules are ISO 6410: minor at 0.8 x major, a thread-length limit line across the
major, a three-quarter circle in the end view, and an internal thread whose
section hatch runs to the major diameter.

standards/{undercuts,rings,centres,orings}.py ship DIN 509, DIN 471/472,
DIN 332-1 and ISO 3601-2 with their SOURCE, their row-key contract and an
**empty** row set. That is deliberate and is the track's table rule taken at its
word: a row that could not be verified against the standard is not shipped,
because a wrong relief depth or groove diameter is a wrong workshop drawing
while a narrow table is merely narrow. Every lookup is refused by name and says
what to transcribe and where; the features accept explicit dimensions so the
geometry is fully exercised, and DIN 332 form R is refused outright rather than
approximated by the straight 60 degree cone of forms A and B. Filling any of
these tables later is a data edit with no code change, and a shape test already
fails any row that lands without a column the drawing code reads.

Three deviations from the plan text, each measured rather than assumed
(the merge agent should keep exactly one copy of the first):

1. standards/__init__.py (Task 1, on main) refused a registration with no rows
   ("registered no rows"), which would have made the four not-transcribed
   tables impossible to register at all - and registering them is what keeps a
   later transcription a data edit. register() now refuses a rows argument that
   is not a dict and accepts an empty one; two tests in tests/test_mech_standards.py
   pin both halves. No existing test asserted the old refusal.

2. ORingGroove's housing kind is carried by `housing`, not `kind`. A dataclass
   field named `kind` overwrites the discriminator _Base.to_dict() writes:
   measured, Probe(id='og').to_dict() == {'kind': 'radial', 'id': 'og'}, so
   feature_from_dict would then refuse its own payload as an unknown feature
   kind and no part carrying an O-ring groove could round-trip through XDATA.
   A new test pins the round trip.

3. The plan's end-chamfer test asserted after == (75.0, 10.0) - the corner
   itself - and before == (72.0, 8.268), a point on neither surface. The real
   edit is before (72.0, 10.0) on the cylinder and after (75.0, 8.268) on the
   end face, consistent with the 45-degree start chamfer the same file pins.
   The test was corrected to the measured geometry; the behaviour is unchanged.

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

engineering/mech/__init__.py

@@ -6,6 +6,31 @@ and alphabetical — keep every side's names.
 
 from __future__ import annotations
 
+from .features import (
+    FEATURE_KINDS,
+    AxialBore,
+    BendLine,
+    CentreHole,
+    Chamfer,
+    CornerChamfer,
+    CornerFillet,
+    CrossHole,
+    Feature,
+    Fillet,
+    GearTeeth,
+    Hole,
+    HoleCircle,
+    HolePattern,
+    Keyway,
+    ORingGroove,
+    Pocket,
+    RetainingGroove,
+    Slot,
+    Thread,
+    Undercut,
+    feature_from_dict,
+    mirror_about_x,
+)
 from .part import (
     PART_KINDS,
     PrismaticPart,
@@ -44,26 +69,49 @@ from .primitives import (
 )
 
 __all__ = [
+    "FEATURE_KINDS",
     "PART_KINDS",
     "ROLES",
     "ROLE_LAYER",
     "Arc",
+    "AxialBore",
+    "BendLine",
+    "CentreHole",
+    "Chamfer",
     "Circle",
+    "CornerChamfer",
+    "CornerFillet",
+    "CrossHole",
     "DimIntent",
+    "Feature",
+    "Fillet",
+    "GearTeeth",
     "HatchArea",
+    "Hole",
+    "HoleCircle",
+    "HolePattern",
+    "Keyway",
     "Line",
+    "ORingGroove",
+    "Pocket",
     "Poly",
     "Prim",
     "PrismaticPart",
     "Pt",
+    "RetainingGroove",
     "RevolvedPart",
     "Role",
     "Segment",
+    "Slot",
     "Text",
+    "Thread",
+    "Undercut",
     "bbox",
     "build_part",
+    "feature_from_dict",
     "inner_radius_at",
     "layer_for",
+    "mirror_about_x",
     "outer_radius_at",
     "outline_bbox",
     "part_length",