feat(Space): add fundamental solution Laplacian theorem

[Lemmy00]

This is a follow-up to #1143 and #1169.

It adds the critical norm-power theorem for the distributional Laplacian in dimensions at least three:

Space.distLaplacian_fundamentalSolution_norm_zpow

The proof reuses the existing norm-power gradient theorem and the distDiv_inv_pow_eq_dim Dirac-delta divergence theorem, so this PR does not add new integration infrastructure or refactor the spherical-coordinate API.

Verification:

• lake env lean Physlib/SpaceAndTime/Space/Norm.lean
• lake build Physlib.SpaceAndTime.Space.Norm
• lake build Physlib
• lake exe runPhyslibLinters
• ./scripts/lint-style.py Physlib/SpaceAndTime/Space/Norm.lean
• git diff --check

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[jstoobysmith]

Looks good - approved.