docs(giga): rework Sei Giga pages for whitepaper v2.0 - #42

A large, well-sourced rewrite of the three Sei Giga pages that genuinely improves claim hygiene, with 14 new theme-aware SVG diagrams; all internal links and anchors I checked resolve. Two blocking issues: the hand-edited llms*.txt artifacts still ship the exact "world's first / Fastest EVM" claims this PR exists to remove (and hand-editing them violates AGENTS.md), and the node-operators.mdx edit deletes the "both default to off" caution while only the Giga Executor default actually flipped.