fix: preserve YAML project cards regardless of key order

[Vilin97]

Auto-rebase silently dropped newly added project registrations when a YAML card began with title: rather than slug:. Parse the YAML sequence and use its source positions to preserve each card verbatim, independently of key order, quoting, or indentation. Reject ambiguous identities and unsupported layouts before writing a merged registry.

Validation: all 450 Python tests pass, including 35 rebase tests covering the reported loss, quoted identities, standalone sequence markers, nested lists/block scalars, duplicate and missing slugs, and byte-for-byte coverage of the actual project registry. Ruff lint, formatting, and git diff --check pass.

The fixed parser also round-trips all 46 currently labeled PR registries without losing or rewriting any card.

[greptile-apps]

Advisory cross-file review against Lean Pool's supervisory rubric. This is not by itself a significance, source-verification, or merge verdict.

[Medium risk] Changes how YAML project cards are parsed and split.

The PR appears safe to merge; no new actionable issue was identified.

Summary

The PR replaces first-key-based card detection with YAML sequence boundaries, preserving cards with reordered keys or standalone sequence markers. It adds regression tests for those layouts and for the real registry.

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