Docs: add source-backed Korean mixed-script UI examples

[bitjaru]

Why

Roadmap issue #8 covers Korean typography and UX writing broadly. This is one small, reviewable slice: examples where Hangul, Latin product names, numerals, money, and units appear in the same UI string.

Scope

Extend engine/UX-WRITING.md with a short mixed-script section containing at least three source-backed before/after examples, such as:

• Hangul copy with an English product or plan name
• Korean currency or percentage text with tabular numerals
• A CTA or status message containing a number and unit

Explain the reusable decision behind each example. Do not turn one product's house style into a universal rule.

Acceptance criteria

• Each example has Do / Don't / Why.
• At least one primary Korean-language typography or UX-writing source is linked.
• Applicability and one counterexample are stated.
• The existing Toss-derived section remains clearly attributed.
• Run node demo-pricing/scripts/build-llms.mjs, node scripts/validate-engine.mjs, and git diff --check.
• Commit any regenerated mirrors produced by the maintained-source change.

Boundary

This issue does not need to solve Hangul CPL ranges or every item in #8. Link the PR to #8 without closing the parent roadmap issue.

Korean-speaking first-time contributors are especially welcome.