docs: merge duplicate networking namespace dirs; guard against recurrence

Documentation namespace cleanup (issue #38).

Audit outcome:

- `date` vs `datetime` are NOT duplicates — `Date` is calendar days since the
  epoch, `DateTime` is instants (seconds); distinct runtime namespaces. Kept
  both.
- `network` vs `networking` WAS a real duplicate. Every other stdlib area
  documents only its namespace (`World.*`, `Screen.*`, …), never the bare
  builtins it lowers to. Networking alone also documented the low-level
  `net_*`/builtin forms under `networking/`, duplicating the `Network.*`
  pages under `network/`. Removed `networking/`; `network/` (the `Network`
  namespace, which the compiler and LSP both expose) is canonical. Folded the
  `@Sync`/`@Owned` framing into `network/_section.md` so no context is lost.
- Dropped the `networking` key from docgen inventory.json.

Guard (AC3): `tools/docgen/check.py` now fails if any `ns:` is documented from
more than one directory, or if two sections share an id or (case-folded)
title — so a duplicate-namespace split cannot silently reappear.

`gen.py` + `check.py` pass (34 sections, 365 symbols).

Closes #38

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/docgen/inventory.json

@@ -187,17 +187,6 @@
     "network-is_owner",
     "network-local_id"
   ],
-  "networking": [
-    "fn-net_send",
-    "fn-net_poll",
-    "fn-owner",
-    "fn-set_owner",
-    "fn-is_server",
-    "fn-is_owner",
-    "fn-local_id",
-    "fn-serialize",
-    "fn-apply"
-  ],
   "operators": [
     "op-arith",
     "op-compare",
@@ -417,4 +406,4 @@
     "crypto-hex",
     "crypto-ct_equal"
   ]
-}
+}