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/standards/rings.py

@@ -0,0 +1,60 @@
+"""DIN 471 / DIN 472 retaining-ring groove dimensions.
+
+SOURCE
+------
+DIN 471:2011-04, *Retaining rings for shafts*, Table 1 (groove diameter d2 and
+groove width m) and DIN 472:2011-04, *Retaining rings for bores*, Table 1.
+
+COVERAGE
+--------
+**No rows are transcribed in this build**, for the reason given in
+``undercuts.py``: the groove diameter is the number a wrong drawing gets wrong,
+and it is not written here from memory. Every lookup is refused by name.
+
+Extending is a data edit: fill ``ROWS_471`` / ``ROWS_472`` with
+``{nominal_shaft_or_bore_mm: {"m": groove_width_mm, "d2": groove_diameter_mm}}``
+and widen the coverage. Those two keys are the contract group S's
+``std_feature_draw`` already reads.
+"""
+
+from __future__ import annotations
+
+from engineering.mech.standards import Coverage, register
+
+SOURCE = (
+    "DIN 471:2011-04 Table 1 (shafts) and DIN 472:2011-04 Table 1 (bores) "
+    "- NOT TRANSCRIBED in this build"
+)
+
+#: The keys every DIN 471/472 row must carry.
+RING_ROW_KEYS: tuple[str, ...] = ("m", "d2")
+
+ROWS_471: dict[float, dict] = {}
+ROWS_472: dict[float, dict] = {}
+
+COVERAGE_471 = Coverage(0.0, 0.0, "mm")
+COVERAGE_472 = Coverage(0.0, 0.0, "mm")
+
+register("DIN 471", ROWS_471, COVERAGE_471, SOURCE)
+register("DIN 472", ROWS_472, COVERAGE_472, SOURCE)
+
+_ROWS = {"DIN 471": ROWS_471, "DIN 472": ROWS_472}
+
+
+def ring_groove_dims(standard: str, d: float) -> dict:
+    """``{"m": ..., "d2": ...}`` for a nominal size, or a named refusal."""
+    key = " ".join(str(standard or "").split()).upper()
+    if key not in _ROWS:
+        raise ValueError(f"retaining-ring groove: expected DIN 471 or DIN 472, got {standard!r}.")
+    rows = _ROWS[key]
+    if not rows:
+        raise ValueError(
+            f"{key}: no rows are transcribed in this build, so no groove can be looked up "
+            f"for d = {float(d):g} mm. Pass m and d2 explicitly, or transcribe "
+            f"{SOURCE.split(' - ')[0]} into engineering/mech/standards/rings.py. "
+            "Nothing is interpolated."
+        )
+    size = float(d)
+    if size in rows:
+        return {name: float(rows[size][name]) for name in RING_ROW_KEYS}
+    raise ValueError(f"{key}: d = {size:g} mm has no transcribed row (sizes: {sorted(rows)}).")