Add assemble_lumped_mass! primitive

[lxmota]

Summary

Adds assemble_lumped_mass! — a partition-of-unity row-sum lumped mass primitive computed directly via per-element scatter of density * N[a] * JxW * I_NDIM, into the assembler's full-DOF residual storage, with free-DOF unknowns extracted for non-condensed mode.

This is distinct from the existing assembly primitives:

• assemble_mass! builds the full sparse consistent mass matrix.
• assemble_diagonal!(asm, mass, ...) extracts the diagonal of the consistent mass (rho * integral N_a^2 dV), which is not partition of unity.
• assemble_matrix_action!(asm, mass, U_zeros, ones_free, p) silently under-counts contributions from constrained columns and breaks partition of unity at free DOFs adjacent to Dirichlet boundaries.

Why this primitive

This is the right primitive for:

• Explicit central-difference dynamics (a = M^-1 f), where partition of unity (total mass = density × volume) is required to recover correct wave speeds at free DOFs adjacent to a Dirichlet boundary.
• Mass-diagonal Jacobi preconditioning, for the same SPD reason.

The row-sum-of-reduced-M approach used previously by some downstream code (Carina.jl) under-counts mass at boundary-adjacent DOFs because M_red * 1_free misses the M_{free,BC} cross terms.

Implementation notes

Reuses the existing assemble_vector! scatter machinery via the AssembledVector return type, so GPU support is inherited automatically (fec_atomic_add! over an H1Field on the target backend, no CPU detour).

Exports:

• assemble_lumped_mass!(assembler, func, Uu, p) — assembly entry point, mirrors the assemble_diagonal! calling convention.
• lumped_mass(asm) — accessor returning free-DOF unknowns (non-condensed) or full-DOF (condensed).

[codecov]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.59%. Comparing base (fd5f098) to head (a957563).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/assemblers/LumpedMass.jl	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #306      +/-   ##
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- Coverage   66.33%   65.59%   -0.75%     
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  Files          55       56       +1     
  Lines        4848     4868      +20     
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- Hits         3216     3193      -23     
- Misses       1632     1675      +43     

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