Publish the dev-tag discovery upgrade to main on a clean release branch

This release branch snapshots the latest dev work onto current main so the
update center can surface recent non-stable Docker tags without operators
looking them up manually. The branch keeps stable Docker discovery as the
primary path while carrying the new recent-tag auto-discovery, UI affordances,
and regression coverage through the repo's release flow.

Constraint: Publication must start from latest origin/main and merge origin/dev on a clean release branch
Constraint: Stable Docker Hub discovery semantics must remain intact while adding recent non-stable discovery
Rejected: Re-open or reuse the already-merged PR #494 branch | it has already been absorbed by main and deleted remotely
Rejected: Publish directly from dev | repo workflow requires a clean release branch to main
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep future dev-tag publication work on fresh release branches cut from origin/main; do not revive merged release branches
Tested: Focused update-center vitest suite, npm run typecheck, npm run repo:drift-check, npm run build:web, npm run docs:build, git diff --check (release branch)
Not-tested: Full npm test remains red because tmp/sub2api frontend suites are still broken in this repo snapshot

docs/k3s-update-center.md

@@ -310,8 +310,9 @@ helper 端使用：
 
 补充一点：
 
-- `Docker Hub` 里的**自动发现**仍然优先 `latest` / `main` / 稳定 SemVer 标签
-- 如果你要部署 `dev`、分支名、临时标签或 `sha-*` 这类非稳定标签，直接在 Docker Hub 卡片底部用“手动部署 Docker Hub 标签”填写即可
+- `Docker Hub` 里的**主候选**仍然优先 `latest` / `main` / 稳定 SemVer 标签
+- 页面还会自动列出最近推送的 `dev`、分支名、临时标签或 `sha-*` 这类**非稳定标签**，并一起带出 digest
+- 如果你要部署一个更老或更特殊、没出现在自动列表里的标签，仍然可以在 Docker Hub 卡片底部用“手动部署 Docker Hub 标签”填写
 
 对已经跑起来的 K3s / Helm 用户来说，更新中心的日常配置主要就在这一页：
 
@@ -330,7 +331,7 @@ helper 端使用：
    - 版本来源发现了可部署版本
    - 如果 Docker Hub 显示的是 `latest @ sha256:...`，说明页面已经识别到 alias tag 当前指向的具体镜像 digest
    - Deploy Helper 显示健康
-4. 如果你要跟随稳定候选，直接点部署按钮；如果你要切到 `dev` / 分支 / 临时标签，就在 Docker Hub 卡片底部手动填写 tag，必要时连 digest 一起填
+4. 如果你要跟随稳定候选，直接点部署按钮；如果你要切到最近的 `dev` / 分支 / 临时标签，可以直接点自动列出的那一项；只有更特殊的 tag 才需要手动填写，必要时连 digest 一起填
 5. 在页面下方看部署日志
 6. 如果升级后发现问题，可以直接在“回退历史”里点旧 revision 回滚；只要该 revision 当时记录了 digest，就会跟着一起回到对应镜像