fix(mv): wrap norm() in Abs for even-grade multivectors

[utiberious]

Summary

Fixes norm() to always return a nonnegative value by wrapping the result in Abs when the multivector has even-grade components (where the norm-squared can be negative, e.g. for timelike vectors in Minkowski space).

Previously, norm() could return expressions with no absolute value bars for even grades, which is mathematically incorrect since the norm should always be nonnegative.

Fixes #522

Changes

• galgebra/mv.py: Wrap norm() result in Abs() when the squared norm may be negative
• test/test_mv.py: Add test verifying norm() returns nonneg for even-grade multivectors
• examples/ipython/test_gsg_undual_etc.ipynb: Update notebook outputs for norm/abs changes

Test plan

• Unit tests pass
• Full CI with nbval notebooks passes

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[utensil]

Thanks for addressing this.

Two requests:

1. Dead code. The if result.is_number: return Abs(result) branch is identical to the final fallback — both return Abs(result). Remove it.

2. Test coverage. The test only covers Euclidean G3. Issue #522 (and PR #510) specifically call out non-Euclidean metrics as the hard case. Please add:

# non-Euclidean: symbolic vector where norm may not be provably nonneg
g2mn = Ga('e', g=[1, -1], coords=symbols('x t', real=True))
v = g2mn.mv('v', 'vector')
assert v.norm(hint='+').is_nonnegative

[utiberious]

Addressed both:

1. Removed the dead if result.is_number: return Abs(result) branch (identical to the fallback).
2. Added non-Euclidean test: Ga('e', g=[1, -1]) with symbolic vector, asserts v.norm(hint='+').is_nonnegative.

Pushed in a16d314.