refactor(home): 首页三字段默认值收敛到 SITE_DEFAULTS（5g T4 补）

- SITE_DEFAULTS 新增 homeProjectsLimit=6 / homeAnnouncement(空) /
  homePostsTracks(全开)，访问器改为引用默认常量而非散落字面量
- 旧 yaml（无三字段）零回归实测：公告不渲染、项目墙默认 6 条、三轨全开

验证: 构建 48 页 exit 0；E2E 37/37（快照口径）

Spec: 2026-11-02_blog-5g-pages (T3/T4 补充)

Co-authored-by: GLM-5.3

src/lib/site-content.ts

@@ -12,6 +12,12 @@ export interface Bi {
 
 /** 与仓库现状逐字一致的默认文案：yaml 全空时产物不变（零回归可视验证锚点） */
 export const SITE_DEFAULTS = {
+  /** 首页项目墙条数（旧 yaml 无此字段时的默认：6 条） */
+  homeProjectsLimit: 6,
+  /** 首页公告（默认空 = 不渲染） */
+  homeAnnouncement: { zh: '', en: '' },
+  /** 首页轨细分（默认全开 = 现状） */
+  homePostsTracks: { reading: true, tech: true, photo: true },
   homeTagline: {
     zh: '写作者 · 读者 · 快门按压者，拥抱开源。',
     en: 'Writer · Reader · Shutter-presser — embracing open source.',
@@ -256,12 +262,12 @@ export async function homeConfig(locale: Locale): Promise<HomeConfig> {
   const d = entry?.data;
   return {
     subtitle: pickBi(d?.subtitle, { zh: '', en: '' }, locale),
-    announcement: pickBi(d?.announcement, { zh: '', en: '' }, locale),
-    projectsLimit: clampInt(d?.projectsLimit, 1, 99, 6),
+    announcement: pickBi(d?.announcement, SITE_DEFAULTS.homeAnnouncement, locale),
+    projectsLimit: clampInt(d?.projectsLimit, 1, 99, SITE_DEFAULTS.homeProjectsLimit),
     postsTracks: {
-      reading: d?.postsTracks?.reading ?? true,
-      tech: d?.postsTracks?.tech ?? true,
-      photo: d?.postsTracks?.photo ?? true,
+      reading: d?.postsTracks?.reading ?? SITE_DEFAULTS.homePostsTracks.reading,
+      tech: d?.postsTracks?.tech ?? SITE_DEFAULTS.homePostsTracks.tech,
+      photo: d?.postsTracks?.photo ?? SITE_DEFAULTS.homePostsTracks.photo,
     },
     latestPhotos: clampInt(d?.latestPhotos, 0, 6, 4),
     perTrackLimit: clampInt(d?.perTrackLimit, 4, 8, 6),