fix(docs-site): publish function pages under /reference/

Cloudflare Pages reserves a top-level `functions/` directory in the deploy
output for Pages Functions (server-side handlers) and strips it from the
static-asset upload. The wrangler log for run 812 shows the collision
exactly: 159 files uploaded, which is precisely the number of files in
dist/ outside functions/ — all 108 files under it were dropped.

Nothing failed loudly. Astro built all 120 pages, wrangler reported
success, and the site worked under `astro preview`; only the deployed
copy 404'd on every function entry and category index.

Move the generated tree to /reference/ and add a test asserting the site
output never emits a name Pages reserves.

docs/build-manual.py

@@ -279,7 +279,14 @@ def build_site(root: Path, out: Path) -> list[dict]:
             continue
 
         # Section 5: category index page keeps its slot; entries explode.
-        slug_dir = "functions"
+        #
+        # Deliberately NOT "functions": Cloudflare Pages reserves a top-level
+        # `functions/` directory in the deploy output for Pages Functions
+        # (server-side handlers) and silently drops it from the static-asset
+        # upload. The pages build fine and never arrive — every entry 404s in
+        # production while working locally. test_site_avoids_reserved_dir
+        # guards this.
+        slug_dir = "reference"
         if rel.name == "index.md":
             target = out / slug_dir / "index.md"
             target.parent.mkdir(parents=True, exist_ok=True)