fix(sync): preserve BOMs and distinguish diagnostics from blockers

[callumalpass]

Summary

• Preserve BOMs in the Obsidian binary-to-text mirror adapter.
• Use the engine plan's blocking flags/count for messaging: nonblocking frontmatter diagnostics are optional review, not mandatory repair or a claimed sync pause.
• Test that missing required fields, wrong types and enum errors stay visible while exact records replicate into a second mocked Vault.
• Add an explicit strict candidate qualification harness; no production option is introduced.

Evidence

• 96 tests, lint, build and unchanged mobile budgets pass with released beta.91.
• The same 96 tests and build/mobile gates pass with immutable Connect candidate 87e28391233c (fix(sync): replicate malformed frontmatter losslessly by default mdbase-connect#436).
• Candidate qualification passes ten exact malformed/valid-BOM/body-only/CRLF round trips through the real Obsidian adapter and portable SDK with mocked Vaults and MemoryAuthority. The released beta.91 control fails at the expected seven-blockers assertion.

Release boundary

This does not yet ship the SDK policy change. Dependency pins and the checked-in generated bundle remain on released beta.91. Advance both SDK pins/lockfile after the Connect change is released; do not install a nonexistent registry version or commit machine-local artifact paths.

Real Obsidian → hosted → second mirror acceptance is still outstanding: no existing vault metadata, installed plugin, LAB service, staging or production was modified. See docs/lossless-sdk-acceptance.md for commands, evidence and blockers.

Separate from recovery PR #12.