chore: replace OLD red SeiLogo banner in README with new 2026 Sei lockup

[alexander-sei]

Replace OLD red SeiLogo banner with new 2026 Sei lockup

The README.md banner at assets/SeiLogo.png is currently the OLD red
Sei mark (231x231, dominant fill ≈ #C3384A, RGB 195,56,74). This PR
replaces it with the official Sei lockup from the 2026 brand kit,
rendered as 600x231 (mark + wordmark, black on transparent — the
on-light variant from the brand kit, suited to GitHub's white README
background).

The README line that consumes it (![Banner!](assets/SeiLogo.png)) does
not need to change.

What changed

• assets/SeiLogo.png — 231x231 OLD red mark → 600x231 NEW lockup (black on transparent).
  Height preserved to keep the banner roughly the same visual size on the README.

Verification

• New PNG: 600x231 RGBA, dominant non-transparent color is #000000
  (the brand-kit on-light lockup; mark + wordmark).
• No remaining #9E1F19 / #9F1912 / #C1121F / #780000 pixels.
• Sized to fit the original 231-pixel height; aspect change is from a square
  symbol to a wide lockup which is the modern brand-consistent presentation.

Repo-wide scan

grep -rIniE '9E1F19|9F1912|9C0910|C1121F|780000|SeiLogo|sei-logo|sei-symbol'
against the repo tree (via the GitHub trees API; the repo is too large to
clone in this environment) found exactly two logo files:

• assets/SeiLogo.png — this PR.
• sei-db/db_engine/litt/docs/resources/littdb-logo.png — unrelated
  (LittDB), not in scope.

No other matches.

Brand references

• Full brand kit: https://brand.sei.io
• 2026 rebrand announcement: https://blog.sei.io/sei-2026-rebrand
  (placeholder URL — update if a different announcement URL is preferred)

Alternatives considered

Two options were viable:

1. Lockup (chosen): rasterize sei-lockup-on-light.svg at 600x231.
   Matches the banner role of the README image; brand-consistent.
2. Symbol at 231x231: drop in the maroon #600014 symbol.
   Preserves dimensions exactly but loses the wordmark.

If reviewers prefer the symbol variant, the alternate PNG is available in
the brand kit at SeiBrandAssets/Logomark/SeiToken.png.

[cursor]

PR Summary

Low Risk
Documentation-only asset swap with no runtime or security impact.

Overview
Updates the README hero image by replacing assets/SeiLogo.png with the 2026 Sei lockup (wide mark + wordmark, black on transparent) instead of the old square red mark.

Dimensions move from 231×231 to 600×231 while keeping roughly the same banner height; the ![Banner!](assets/SeiLogo.png) line in README.md is unchanged. No application, chain, or dependency code is touched.

^{Reviewed by Cursor Bugbot for commit ce59e9a. Bugbot is set up for automated code reviews on this repo. Configure here.}