✅ test(spec): the Fields tables must name fields that exist

[nstarman]

The follow-up I flagged on #762. Three spec sections have now been found describing fields
the class does not have — Scale (#750), Shear (#762), and Tangent below — every one
by accident rather than by looking. This closes the class.

Every !!! info section with a Fields block is now checked against
dataclasses.fields. Ten sections qualify, out of 140.

It found a fifth stale entry immediately

Tangent documented a manifold field that has never existed —
dataclasses.fields(Tangent) is (data, chart, basis, semantic, frame). The manifold is
reached through M, a property derived from the chart.

The same section's "Post-init checks" were wrong in two more ways: there is no
__post_init__ on Tangent at all, and the check it names does not exist either.

def __check_init__(self) -> None:          # not __post_init__
    self.M.check_chart(self.chart)         # not manifold.has_chart(chart)
    self.chart.check_data(self.data, keys=True)

Corrected, including the note that M is derived rather than stored — which is exactly
the fact the phantom row obscured.

Two deliberate limits

Sets, not sequences. Translate looked like a second defect: the spec orders its
fields (delta, chart, semantic_kind, right_add) while dataclasses.fields gives
(delta, chart, right_add, semantic_kind). It has a custom __init__, and the spec
matches the constructor — the order a reader would actually use. Enforcing
dataclasses.fields order would have failed a section that is right, so only membership
is checked.

Per-type sections only. The exported-objects table earlier in the spec is a curated
summary — the coordinax.charts row omits 50-odd class names on purpose — so it is not an
__all__ mirror and is deliberately untouched. (I nearly built that check during #730
before noticing.)

The test can actually fail

Verified in both directions rather than assumed:

spec claims a field the class lacks  -> Tangent: spec documents field(s) the class
                                        does not have: ['manifold']
class has a field the spec omits     -> Linear: spec documents field(s) the class
                                        does not have: ['grup']

Both spellings of a Fields block are parsed — bullet list and markdown table, both in use
— and a guard test asserts the parser still finds at least eight sections, since a regex
that quietly stops matching would pass forever and prove nothing.

tests/unit + docs: 3868 passed, 5 skipped.

[Copilot]

Pull request overview

Adds a regression test to keep docs/spec.md’s per-type Fields blocks in sync with actual dataclass fields, preventing spec drift in documented public APIs. Updates the Tangent spec section to remove a non-existent manifold field and to describe the correct init-time validation behavior (M derived from the chart; checks performed in __check_init__).

Changes:

• Introduce a unit test that parses docs/spec.md !!! info sections with Fields blocks and asserts set equality with dataclasses.fields(...).
• Fix the Tangent spec section by removing the phantom manifold field and correcting the init-checks description.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/unit/test_spec_fields_match_code.py	New test that enforces Fields blocks in the spec match real dataclass fields (membership, not order).
docs/spec.md	Correct Tangent documentation: remove non-field manifold, clarify M is derived, and fix init-checks text.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.66%. Comparing base (ff1167a) to head (924f9e1).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #768      +/-   ##
==========================================
+ Coverage   96.57%   96.66%   +0.08%     
==========================================
  Files         267      267              
  Lines        8963     9079     +116     
==========================================
+ Hits         8656     8776     +120     
+ Misses        307      303       -4     

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