fix: align Lorentz/planar coordinate conventions across signatures

[henryiii]

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Summary

Several _compute functions returned different results for the same vector depending on which coordinate system it was stored in. This PR aligns them so all 12 Lorentz coordinate systems (and the planar/spatial helpers) agree, following the project convention of matching ROOT's TLorentzVector / Math::LorentzVector.

Each fix was reproduced before being changed; regression tests cover both the T- and Tau-coordinate representations.

Bugs fixed

1. Et sign convention differed by signature. xy_z_t/xy_z_tau returned sqrt(Et2) (always ≥0) while the theta/eta/rhophi variants returned t·sinθ (sign-preserving in t). E.g. vector.obj(px=3, py=4, pz=0, E=-13).Et gave +13 but vector.obj(pt=5, phi=0, pz=0, E=-13).Et gave -13. Now xy_z_* use copysign(sqrt(Et2), t), matching ROOT's TLorentzVector::Et() (E<0 ? -sqrt(Et2) : sqrt(Et2)).

2. Mt/Mt2 disagreed between T and Tau coordinates. The T-variants computed t²−z² unclamped (so Mt=nan for transverse-spacelike vectors), while the Tau-variants clamped with maximum(…, 0) (giving Mt=0). E.g. vector.obj(px=3, py=4, pz=10, E=2).Mt2 == -96, .Mt == nan, but the equivalent tau vector gave 0.0. Now both representations return the signed Mt2 = t²−z² (the Tau-variants reconstruct it as tau2 + rho², where tau2 is the signed copysign(tau², tau)), and Mt = copysign(sqrt(|Mt2|), Mt2), matching ROOT's Mt().

3. Lorentz scale flipped tau's sign for negative factors. The Tau-variants returned tau * factor; since negative tau encodes spacelike, scaling a timelike vector by −2 produced a spacelike encoding, disagreeing with the T-coordinate version. Now tau * |factor|: scaling by λ multiplies t²−mag² by λ², so |tau| scales by |λ| and the causal character is invariant. (spatial/scale.py already takes |factor| for rho.)

4. Planar unit of the zero vector was inconsistent. xy used nan_to_num(…, nan=0) (zero → (0,0)) but rhophi returned (1, phi) unconditionally. rhophi now mirrors spatial/unit.py: rho=0 → 0.

5. spatial/eta.py inconsistent nan guard. xy_theta wrapped -log(tan(θ/2)) in nan_to_num(nan=0.0, …) but rhophi_theta returned the raw expression (giving nan for θ outside (0, π)). The same guard was added to rhophi_theta.

All changes stay as array-safe lib.* expressions, so the numba (register_jitable), awkward, and sympy backends keep working (copysign/maximum/nan_to_num have sympy shims). Two existing scale tau-coordinate test assertions and the sympy Mt _t assertions encoded the previous inconsistent behavior and were updated to the now-consistent values.

Convention decisions for maintainer review

These are physics-convention choices — please confirm they match the intended ROOT semantics:

• Et is sign-preserving in energy (negative E → negative Et), per ROOT TLorentzVector::Et(). Applied uniformly to all 12 signatures.
• Mt/Mt2 are signed (Mt2 = t²−z² unclamped; Mt = copysign(sqrt(|Mt2|), Mt2)), per ROOT Mt(), so transverse-spacelike vectors give a negative Mt rather than nan/0. The previous Tau-variant clamp to 0 is removed.
• scale preserves causal character: |tau| scales by |factor|, sign unchanged. A side effect: for vectors stored in Tau coordinates, (v * negative).t now reports the (always-positive) reconstructed |t| with the correct timelike/spacelike magnitude instead of the previous sign-flipped value — the two updated assertions in tests/compute/test_scale.py reflect this.
• Zero-vector unit maps to the zero vector in both xy and rhophi planar representations (matching the spatial backend).

Note: a Tau-coordinate vector does not store the sign of t, so (v*negative).t reconstructed from tau is always non-negative; this PR makes its magnitude consistent across coordinate systems but does not attempt to recover the lost sign (out of scope).

Tests

Added regression cases for: negative-energy Et across all signatures; transverse-spacelike Mt/Mt2 across both T- and Tau-coordinate vectors; negative scale factor on Tau-coordinates (timelike and spacelike); zero-vector planar unit in rhophi (object + numpy); and eta for θ outside (0, π) in both xy_theta and rhophi_theta.

uv run pytest tests/compute/ tests/backends/test_object.py tests/backends/test_numpy.py tests/backends/test_sympy.py tests/backends/test_awkward.py tests/backends/test_numba_object.py — all green.

Part of #711

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.18%. Comparing base (d9ef1db) to head (09245bb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #717      +/-   ##
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+ Coverage   87.16%   87.18%   +0.01%     
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  Files          96       96              
  Lines       11199    11211      +12     
==========================================
+ Hits         9762     9774      +12     
  Misses       1437     1437              

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