refactor(stdlib): de-abbreviate namespaces (Mem/Sys/Net/Collide)

Expand the abbreviated public namespaces to full words, part of the
language-wide de-abbreviation pass:
  Mem -> Memory, Sys -> System, Net -> Network, Collide -> Collision

Math stays (universally accepted, like int/bool). Renames the dispatch
strings, LSP signatures, docs (dirs, files, frontmatter), and the
inventory manifest; behavior is byte-identical (the bare rt_ targets are
unchanged). Reseeded; C-free bootstrap fixpoint holds; all suites green.

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

docs/language/net/net-serialize.md

@@ -1,14 +0,0 @@
----
-id: net-serialize
-name: Net.serialize
-category: net
-kind: namespace-method
-tokens: Net.serialize
-sig: Net.serialize(entity, buf) -> int
-tip: Serialize an entity's synced state.
-order: 2
-ns: Net
-member: serialize
----
-
-Writes the <code>@Sync</code> fields of <code>entity</code> into <code>buf</code> and returns the byte count, for sending over the wire.