Polish terminal focus, cursor, and welcome surface

[sionic-khope]

What changed

• Adds Ghostty-aligned terminal padding, truecolor environment metadata, readable blinking cell cursor, and click-to-focus routing.
• Shows a cyan Miku Code ASCII welcome banner only on the first local launch, preserving restored sessions.
• Keeps Metal and AppKit text geometry aligned and rebases cursor coordinates across paged scrollback.
• Replaces the overly pixelated idle Miku artwork with a smooth antialiased chibi asset.

Verification

• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer swift test (79 passed)
• git diff --check
• Debug app launch and asset alpha inspection passed.

Notes

• Merge intentionally left to the repository owner.
• Automated macOS click injection was unavailable because Accessibility automation returned error -25200; programmatic first-responder and source-level click paths are covered.