[B15] Source-selected finite matter and Spin action

[muellerberndt]

Construct and select a finite matter module from the same source packet as B14, then prove the induced current action, grading, charge characters, and finite Spin data without choosing the Standard Model multiplet from its laboratory target.

Deliverables:

• a frozen source grammar and complete equal-budget matter candidate class;
• a source rule selecting the finite exterior or equivalent matter construction;
• exact weights, multiplicities, faithfulness, chirality grading, anomaly checks, and action of every B14 generator;
• finite Spin or Clifford data and its compatibility with the selected matter action;
• refinement and recharting naturality on the declared finite tower;
• alternative-menu, duplicated/inert-sector, wrong-grading, and target-import controls;
• Lean theorems and deterministic source receipts;
• an assumption manifest separating finite matter from continuum Spin locality, particle poles, families, and laboratory attachment.

A successful exit selects one finite same-source matter action. It does not establish the continuum spin-statistics theorem, physical fermions, particle masses, or a laboratory generation.

Absorbs the executable finite scope of archived #314.
Depends on: B14 (#705).
Wave: V2-W1 Observer laws.

[muellerberndt]

The target-clean matter-class freeze is on record (code/b15_matter_freeze/, validator-replayed and tamper-tested): candidates are nonempty graded complex modules of the complexified semisimple part at each classified compact stratum under the declared dimension budget, zero abelian charges (the only source-certified charge lattice), integral weights, and grading up to global flip. Exact counts per family with the anomaly-zero and carrier-lattice subclasses recorded rather than imposed: the generic mirror strata carry 4967 graded candidates each with 1921 anomaly-zero; the plane stratum carries 10590; mirror equality is forced and checked. Every constraint cites a source receipt or a declared structural budget; faithfulness, anomaly vanishing, lattice integrality, self-duality, irreducibility, and every laboratory quantum number are deliberately left out of membership, keeping the class larger. The frozen observables and barred list separate structural invariants from target data, and the decision rule declares that failure to select is a valid negative exit. Source selection over this frozen class is the open continuation, gated on the same-source current packet.