fix: restore Markdown relationship lines

[teloverge]

What changed

• Parse Blocked by, Blocks, and Parent ATX sections from issue Markdown.
• Merge Markdown-derived relationships with native GitHub relationship data while preserving native parent authority.
• Enrich cached snapshots in memory on load so existing local data recovers without rewriting the cache.
• Ignore fenced examples, indented pseudo-headings, cross-repository references, and malformed #N tokens.
• Document the user-visible repair under Unreleased.

Why

Relationship edges disappeared when GitHub's structured relationship fields were absent even though the issue bodies still contained the repository's established relationship sections. The renderer was working, but the model reaching it had no edges.

Impact

Dependency and parent lines are restored while the constellation remains spatially stable. The fallback is local and additive: it does not add GitHub requests, pagination, or renderer behavior.

Validation

• cargo.exe fmt --all -- --check
• cargo.exe clippy --workspace --all-targets --locked -- -D warnings
• cargo.exe test --workspace --locked -- --test-threads=1
• Frontend: 204 tests passed; focused edge visuals: 6 tests passed
• svelte-check: 0 errors and 0 warnings
• Production web build passed
• Windows-only NSIS build passed after merging current main
• Packaged desktop entry point: Windows GUI subsystem; CLI entry point: Windows console subsystem
• Final standards/spec review and scoped fix re-review: clean