Add an audited local conversion MVP

[playchaodd68]

Summary

• add an isolated henry-mvp wrapper for audited local document-to-Markdown conversion
• report safe provenance and structural metrics without serializing paths or document content
• surface mixed-PDF OCR and encoding warnings instead of reporting false-clean conversions
• enforce private output modes and rollback-safe Markdown/report publication
• pin a repository-local Rust toolchain and add a full-history privacy audit

Why

This turns the library into a reproducible local workflow for converting a confidential document while keeping the source and generated output outside Git. The wrapper also makes partial-conversion risks and output-boundary behavior explicit.

Impact and scope

• the upstream library API remains available as before
• no private source PDF, extracted Markdown, or generated report is committed
• image-only PDFs still fail with a typed OCR-required error
• mixed PDFs retain extractable text and emit safe warnings

Validation

• 208 upstream library tests, 1 robustness test, and 8 snapshot tests passed; 1 local-samples test ignored
• 18 MVP integration tests passed
• locked tests, rustfmt, and Clippy with warnings denied passed
• the real 23-page local conversion completed with source/fixture hash equality
• private directory/file modes and four-commit full-tree privacy audit passed

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