geometric_algebra: deprecate [DFM] conventions, add corrected operations

[inducer]

The operators in this module follow the conventions of Dorst/Fontijne/Mann (GA4CS), which have been shown to rest on shaky foundations; see Eric Lengyel's Poor Foundations in Geometric Algebra. Incorporate the corrections from that article. All existing behavior is preserved and deprecated, not removed.

New operations (validated against independent geometric-product references in the new test):

• MultiVector.inner: the (metric) inner product <A B~>_0, the unique extension of the metric to the full exterior algebra. Scalar, vanishes across grades, and induces the norm (A.inner(A) == A.norm_squared()).
• MultiVector.left_contraction / right_contraction: the corrected interior products <B A~>{grB-grA} and <B~ A>{grA-grB}. For equal-grade blades these reduce to the inner product (the [DFM] contractions reduced to the scalar product), and the geometric product decomposes as aB = B.right_contraction(a) + a^B.
• MultiVector.hodge_dual: the Hodge dual A~ I (equivalently, the right complement of GA). Satisfies A ^ B.hodge_dual() == A.inner(B) * A.I and has a consistent orientation (the [DFM] dual A I^-1 flips, e.g., the dual of e0 in 3D).

Deprecated (DeprecationWarning + doc notes):

• The |, <<, and >> operators. | can produce non-scalars, so it is an interior product rather than an inner product; for k-blades, A | A == (-1)**(k(k-1)/2) * A.inner(A). The [DFM] contractions differ from the corrected ones by a grade-dependent sign.
• scalar_product (the superfluous reversed-Gram-determinant form); use inner instead.
• dual (the [DFM] dualization mapping A I^-1); use hodge_dual instead.

Other changes:

• norm_squared computes A.inner(A) directly (identical values, no longer routed through the deprecated scalar product).

• The module docstring gains a "Conventions and known issues" section (ga-conventions) and a doctest contrasting the new and deprecated operations; the operator table and method docstrings point at the replacements.

• The pre-existing GA test keeps exercising the [DFM] conventions (with a targeted filterwarnings); a new test covers the corrected conventions and their exact relationship to the deprecated operators.

Decisions and trade-offs:

• Backward compatibility comes first: downstream users keep working, but will see deprecation warnings where they use |, <<, >>, scalar_product, or dual. Migration path: | -> inner / left_contraction / right_contraction, dual -> hodge_dual, scalar_product -> inner.
• The new operations are implemented for orthogonal (diagonal-metric) spaces only, consistent with every other product in the module (NotImplementedError otherwise). The general-metric definitions (A^T G B for the inner product, the right complement of GA for the dual) are documented as the natural extension; supporting non-orthogonal or degenerate (e.g. PGA) metrics is a separate, larger effort.
• The article's "clean" Hodge-dual definitions of the contractions (A* vee B) are internally inconsistent as stated; the blade identities <B A~> / <B~ A>, which reduce to the inner product for equal grades and
  fix the vector decomposition, are implemented instead.

Assisted-by: Zed:Qwen3.8-27B-FP8