feat(ui): draw Claude Code's mascot in the welcome overview

[ayush5harma]

Summary

• Draw Claude Code's mascot in the welcome overview in place of the ASCII crab: the three rows of block glyphs from the Claude Code 2.1.293 welcome screen, in its clawd_body colour, with the eye cells on its clawd_background black.
• The logo is nine columns wide where the crab is sixteen, so the overview text column moves from 20 to 13. The width and wrapping tests follow it, and measure columns by display width because each glyph is three bytes.

Why

Sessions in claude-rs run on the stock Claude Code, and the two banners differ for the same session. The rows were read from Claude Code's own screen with its colour escapes, not reconstructed. The eye cells sit on black so that the open quadrant of each eye glyph reads as an eye on light and dark terminal themes alike.

This is a design call, and it's yours to make (see #440). The crab is the project's own identity and the mascot is Anthropic's. This PR is kept separate so it can be declined without affecting anything else.

Closes #440

Validation

• Automated, on this branch (macOS 27.2 on Apple Silicon, rustc 1.98.1 from nixpkgs rather than the 1.89 CI toolchain):
  • cargo fmt --all -- --check: passes
  • cargo clippy --all-targets --all-features -- -D warnings: passes, with five lints new in clippy 1.98 allowed (collapsible_match, manual_is_multiple_of, manual_is_variant_and, map_unwrap_or, while_let_loop). Across all targets, clippy 1.98 reports the same seven warnings on main and on this branch, none of them in this diff.
  • cargo fetch --locked: passes
  • cargo test --locked --all-features: all targets pass (lib 1878, terminal_resize 35 plus a serial run). The only failures were two update_check tests, caused by CLAUDE_RUST_NO_UPDATE_CHECK in my environment; they pass with it unset.
  • npm --prefix agent-sdk test (162) and npm --prefix agent-sdk run lint: pass
  • Not run: cargo deny, the MSRV check, the duplicate-code scan, Linux and Windows. I'm leaving those to CI.
• Manual: started the debug binary against the real Claude Code 2.1.295 in tmux and captured the welcome screen on main and on this branch.
• Screenshot/video (if UI changed): the text of both screens is below. The mascot is drawn in rgb(215, 119, 87), with the eye cells on black.

Before (main):

      _~^~^~_       Version: 0.15.2
  \) /  o o  \ (/   Subscription: Claude Team
    '_   -   _'     Cwd: ~/proj
    / '-----' \     Session ID: 7ca0ea29-...

After:

   ▐▛███▛█   Version: 0.15.2
  ▝▜██████▀  Subscription: Claude Team
   ▝▝   ▝▝   Cwd: ~/proj
             Session ID: 0bd87eaa-...

Notes

• Breaking changes: N/A
• Docs updated: N/A. Suggested changelog entry for the release: Welcome mascot: The welcome overview draws Claude Code's mascot instead of the ASCII crab.
• Governance/release impact: N/A

🤖 Generated with Claude Code

[srothgan]

Pretty sure this violates Anthropic brand guidelines, so this wont be merged https://code.claude.com/docs/en/agent-sdk/overview#branding-guidelines