Add v0.5 submission readiness gate

[ehsanestaji]

Summary

Adds the v0.5 Submission Readiness Gate for FastaGuard's assembly-first FASTA preflight contract.

• Adds --gate submission and --submission-target generic|ncbi.
• Threads submission target metadata through CLI config, gate decisions, provenance, readiness, reports, schema, and compare mode.
• Adds submission readiness output fields for JSON, TSV, HTML, MultiQC, and cohort compare reports.
• Updates golden fixtures, schema contract coverage, examples, evidence notes, release notes, README positioning, and use-case documentation.

Product Boundary

FastaGuard remains a FASTA-level, database-free preflight tool. Passing the submission gate does not guarantee NCBI/ENA/DDBJ acceptance, biological completeness, annotation correctness, or contamination status.

Verification

• cargo test --locked
• python3 -m unittest discover -s tests/python
• git diff --check
• CLI smoke checks for fastaguard --version, --help, --schema, --finding-catalog, single-file --gate submission, and compare-mode --gate submission outputs.