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>

docs/language/network/_section.md

@@ -4,4 +4,4 @@ title: Network
 order: 6
 ---
 
-The low-level networking seam — send and poll datagrams, serialize and apply entity state, and read ownership and role. Offline these collapse to single-player defaults.
+The low-level networking seam — send and poll datagrams, serialize and apply entity state, and read ownership and role. Multiplayer is normally a compile-time property of the ECS, not glue you thread by hand: mark fields <code>@Sync</code> and models <code>@Owned</code> and the compiler generates the replication. These <code>Network.*</code> primitives are what that sugar lowers to, for when you drive the transport yourself. Offline they collapse to single-player defaults.