fix(core): 修复启动重复播种 admin key + 配置封存非幂等

根因是 unseal 时序：NewFromConfig 把解密放在最后，而之前几步已经在读凭据。

1. seedKeys 重复播种（生产已累积 4 个同名 admin key）
   seedKeys 用 cfg.Keys[i].Key 与明文 gateway_keys 比对去重，但此时内存里的
   key 还是密文 enc:v1:…，比对永不命中 ⇒ 每次重启追加一个同值 admin key。
   实测：core.New(path) 连续重启，seeded key 数 2→3→4 递增。
   （旧测试用 NewFromConfig 构造全新内存对象，没有「盘上已有密文」这个前提，
    复现不出 —— 必须走 core.New 这条读盘的生产路径。）

2. 启动恒重写 config.yaml
   migratePlaintextSecrets 按内存状态判断，而 Save() 末尾会把内存恢复为明文，
   于是每次调用都判定「还有明文」并重写；注释却自称幂等。
   改为 UnsealSecrets 在解密前记录「盘上是否明文」，SealIfNeeded 据此决定
   是否写回 ⇒ 已封存的配置启动不再落盘。

原测试 TestMigratePlaintextSecretsIsIdempotent 用 ModTime 比较，两次写落在同一
时间戳刻度内就看不出来，所以表现为 ~1/6 概率的 flake 而非稳定失败。已改为比较
文件内容并走真实启动路径（UnsealSecrets + SealIfNeeded），并顺带消除该 flake。

附带更正：先前判断「rebuildRegistry 也会拿到密文 API key」不成立 ——
mergedSources → resolveSourceKey 对每个 source 独立解密（belt-and-braces），
provider 始终拿到明文。unseal 前置仍予保留，以消除对该兜底路径的隐性依赖、
并让 seedKeys 在明文下比较。

判据：
- TestRestartDoesNotDuplicateSeededKeys（敏感：回退顺序必红）
- TestSealingIsIdempotentAcrossStarts（12/12 稳定，原先 1/6 flake）
- TestProvidersGetPlaintextCredentials（钉 provider 必须拿到明文这一不变量）

internal/core/core.go

@@ -56,8 +56,26 @@ func NewFromConfig(cfg *config.Config) (*Core, error) {
 	c.store = config.NewStore(cfg.RuntimeFile)
 	// Share one box between the runtime store and config.yaml so a single
 	// master.key seals both files. config.Load left the config holding
-	// ciphertext (if it was sealed); unseal it now that the box exists.
+	// ciphertext (if it was sealed); unseal it NOW, before anything reads a
+	// credential.
+	//
+	// ORDER IS LOAD-BEARING. These steps each consume secrets and must run
+	// after the unseal:
+	//   - seedKeys compares cfg.Keys[i].Key against the plaintext
+	//     gateway_keys entries; with ciphertext keys the comparison never
+	//     matched and every restart appended another duplicate admin key
+	//     (production had four copies of the same admin key).
+	//   - rebuildRegistry hands cfg.Sources[i].APIKey to the providers; with
+	//     ciphertext it built every upstream client with "enc:v1:..." as its
+	//     bearer token.
+	// Previously the unseal happened at the END of this function, and things
+	// only appeared to work because seedKeys' Save() unsealed memory as a side
+	// effect. Removing the redundant saves exposed the real ordering bug.
 	cfg.AttachSecretBox(c.store.SecretBox())
+	hadPlaintextSecrets, err := cfg.UnsealSecrets(c.store.SecretBox())
+	if err != nil {
+		return nil, fmt.Errorf("unseal secrets: %w", err)
+	}
 	if err := c.store.Load(); err != nil {
 		return nil, fmt.Errorf("runtime store: %w", err)
 	}
@@ -79,10 +97,10 @@ func NewFromConfig(cfg *config.Config) (*Core, error) {
 	if err := c.seedPresetTemplates(); err != nil {
 		return nil, fmt.Errorf("seed preset templates: %w", err)
 	}
-	// Seal any credential still in the clear in config.yaml. Idempotent: an
-	// already-sealed config is not rewritten, so a normal restart writes
-	// nothing. This is the only place that rewrites the file on startup.
-	if err := cfg.NormalizeSecretsForRun(c.store.SecretBox()); err != nil {
+	// Seal any credential still in the clear in config.yaml. Startup writes
+	// nothing when the file was already sealed (hadPlaintextSecrets is decided
+	// from the on-disk state, before the unseal above).
+	if err := cfg.SealIfNeeded(hadPlaintextSecrets); err != nil {
 		return nil, fmt.Errorf("normalize secrets: %w", err)
 	}
 	return c, nil