prd-7 phase 3: materialize _audit sheet — constraint_score, legal_result, stage_trace per transaction row

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Context

PipelineState<Reconciled> carries confidence, issues: Vec<Issue>, and meta: MetaCtx { accumulated_confidence, stage_trace, flags }. After PRD-7 Phase 0, it also carries legal verification results in issues and constraint evaluations in meta.flags. None of this reaches the workbook — the _audit sheet in crates/ledger-core/src/workbook.rs is a placeholder.

This is the CPA-visible payoff. Without it, the accountant sees no evidence of the verification layer.

Audit Row Schema

One row per committed transaction. Columns:

Column	Source	Type
entry_id	Blake3 of document_id + source_ref	String
constraint_score	ConstraintEvaluation::to_confidence().0	f32
legal_result	first legal_violation issue code or "ok"	String
disposition	worst Disposition across all issues	String
accumulated_confidence	meta.accumulated_confidence	f32
stage_trace_json	serde_json::to_string(&meta.stage_trace)	String
flags	comma-joined MetaFlag display strings	String
invoice_arithmetic_ok	InvoiceVerification::arithmetic_ok if present	bool
commit_gate	CommitGate variant name	String

Implementation Notes

• Add AuditRow struct to crates/ledger-core/src/workbook.rs mirroring the schema above.
• workbook.rs already uses rust_xlsxwriter (0.94.0). The _audit sheet is written after the transactions sheet in the same Workbook write pass.
• stage_trace_json column should be narrow (hidden by default via column width = 0 or a note in the write call) so it doesn't clutter the CPA view.
• flags display: implement std::fmt::Display for MetaFlag in validation.rs.
• commit_gate column: pass the gate into the row builder; derive the variant name via match (no strum needed since the column is write-only).

Acceptance Criteria

• workbook.rs writes an _audit sheet with the nine columns above.
• For a committed transaction with one Z3Result::Violated issue, legal_result column = "legal_violation".
• For a transaction with CommitGate::Approved { confidence: 0.95 }, commit_gate = "Approved".
• stage_trace_json is valid JSON parseable by serde_json.
• Existing workbook tests still pass.

Files

• crates/ledger-core/src/workbook.rs — primary change site
• crates/ledger-core/src/validation.rs — add Display for MetaFlag

Dependencies

Logically follows #55 (TransactionFacts population) so legal_result column has real data. Can be implemented against test-fabricated states first.