fix: split TUI into lazy chunk so prefix never loads natives on Windows

[hyldmo]

Follow-up to #119: farero reported yarn verify (2.19.1, --prefix, true-shorthand config) still segfaults at startup on Windows, fault in ghostty-opentui.node.

Root cause (not the config shape)

Published 2.19.1 dist/numux.js inlines src/ui/app.ts and keeps a top-level from "ghostty-opentui" import. Bun.build without splitting: true emits a single chunk, so the #119 source-level dynamic import was defeated by hoisting: every invocation, including --version, evaluates natives at startup and dies on Windows. The true shorthand is pure string building (lint: true → bun run lint) and never touched natives; my first smoke only ran source, which is why it stayed green.

Changes

• build.ts: splitting: true (+ stable chunk naming). Verified: dist/numux.js has zero ghostty/opentui references; natives live only in lazy chunks behind runtime import().
• scripts/windows-prefix-smoke.ts: every scenario now runs against both src/index.ts and the built bundle (hard-fails if dist is missing), plus a new true-shorthand case with isolated package.json. 28 assertions, green locally on both entries.
• ci.yml: windows job builds before smoking.

Proof pending

Windows dispatch on this branch will show the bundle phase green. After merge, semantic-release cuts the fix version for farero to consume.

[github-actions]

🎉 This PR is included in version 2.19.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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