Improve graph layout: crossing-aware row packing

[danipen]

Summary

Overhauls the Graph tab's layout algorithm to produce a more compact, readable branch graph with fewer crossing lines.

• New crossing-aware row packing (packing.ts): packs each chain onto a row while minimizing vertical-line crossings, then runs a bounded improvement pass (nested-pair swaps + greedy lifts) that reclaims wasted vertical space. Each accepted move strictly shrinks (crossings, total row distance) lexicographically, so the pass converges.
• More compact layout: tightens row assignment so the graph occupies less vertical space.
• Branch labels factored into packing: chains reserve room for their branch labels so labels no longer collide with adjacent lines.
• Keeps layout logic pure and fully unit-tested (layout.test.ts, packing.test.ts).

Testing

• bun run lint — clean
• bun run typecheck — clean
• bun test — 809 pass, 4 skip, 0 fail