feat(01-04): Vite-native content pipeline + Zod schemas + demo fragment + /content/ README

- FragmentSchema with stable-string-ID regex /^season\d+\.[a-z0-9._-]+$/
- SeasonContentSchema wraps fragments[]
- loader.ts uses import.meta.glob with literal patterns (Pitfall 1)
- Throws on schema violation at module-eval time, failing npm run build (PIPE-01)
- Test-only loadFragmentsFromGlob helper for unit-test injection
- Demo fragment season0.demo.first-light proves end-to-end round-trip
- content/README.md documents the convention for Phase 2 writers (STRY-09)
- Removes now-redundant src/content/.gitkeep firewall marker

content/seasons/00-demo/fragments.yaml

@@ -0,0 +1,13 @@
+# /content/seasons/00-demo/fragments.yaml
+#
+# Phase 1 demo fragment — proves the loader round-trips end-to-end.
+# Removed in Phase 2 when real Season 1 content lands under /content/seasons/01-soil/.
+#
+# Fragment ID convention is `season<N>.<id>` per CLAUDE.md "Code Style"
+# and content/README.md. Never numeric. Renames forbidden once shipped.
+fragments:
+  - id: season0.demo.first-light
+    season: 0
+    body: |
+      The garden remembers the first time it was tended,
+      though it cannot say in whose voice.