fix: derive a topic slug that survives a Korean name

Creating a topic failed intermittently — "a topic with that slug already
exists" — and worked when the author retried with a different name. The rule
was never intermittent, only invisible: the slug kept `[a-z0-9]` and dropped
everything else, so a Korean name contributed nothing. Whatever Latin word or
number happened to be in it became the entire slug.

Two ways that goes wrong, and the author hit both. `인증` reduced to an empty
string, which the form refused before a request was ever sent. `Redis 캐시` and
`Redis 클러스터` both reduced to `redis`, so the second one collided with the
first — a real conflict, reported honestly, about a slug the author never chose
and could not see.

Hangul is now romanized rather than discarded. Syllables decompose
arithmetically into initial, medial and final jamo, so this needs no table and
is deterministic: `백엔드 아키텍처` becomes `baekendeu-akitekcheo`. Only the
jamo mapping from Revised Romanization is applied — the sound-change rules are
deliberately left out, because a slug is read, not pronounced, and those rules
would make one name produce different slugs in different contexts.

The output keeps the shape document slugs already use
(`^[a-z0-9]+(?:-[a-z0-9]+)*$`), so the repository has one slug rule rather than
two, and the tests assert exactly that.

src/features/tech-log/presentation/studio/components/taxonomy-manager.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useState, type FormEvent } from "react";
 
 import type { ProjectIndexItem, TopicEdit } from "../../../contracts/management/contract.ts";
+import { slugFromName } from "./slug-from-name.ts";
 import { useStudio } from "../use-studio.ts";
 
 /**
@@ -51,15 +52,11 @@ export function TaxonomyManager() {
   }, [managementGateway, generation]);
 
   /**
-   * slug 를 비워 두면 이름에서 만든다. 한글 이름이 흔한데 slug 는 ASCII 만 받으므로, 비운 채로
-   * 저장하면 서버가 422 로 거절한다 — 사용자가 규칙을 몰라도 되도록 여기서 채운다.
+   * slug 를 비워 두면 이름에서 만든다. 한글 이름이 흔한데 이전 규칙은 한글을 전부 버려서, 이름에
+   * 섞인 영문·숫자만 남았다 — `인증` 은 빈 slug 가 되고 `Redis 캐시` 와 `Redis 클러스터` 는 둘 다
+   * `redis` 가 됐다. 지금은 로마자로 옮긴다 ({@link slugFromName}).
    */
-  const slugify = (value: string) =>
-    value
-      .trim()
-      .toLowerCase()
-      .replace(/[^a-z0-9]+/gu, "-")
-      .replace(/^-+|-+$/gu, "");
+  const slugify = slugFromName;
 
   const submitTopic = async (event: FormEvent) => {
     event.preventDefault();