Add Stage 2 matrix build service boundary

[anth-volk]

Context

PR 2f in the Stage 2 calibration package lane adds a typed service boundary around matrix materialization so standard and chunked builds produce the same contract-safe metadata surface.

Scope

• Introduce MatrixBuildSpec, MatrixBuildResult, and MatrixBuildService around the existing matrix builder engine.
• Emit matrix_summary.json with shape, sparsity, target-order, builder-mode, and chunk lineage metadata.
• Reference the matrix summary from the Stage 2 calibration package contract and pipeline map.
• Add focused unit coverage for matrix summary invariants, contract references, and docs extraction.