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/system/system-arg.md

@@ -0,0 +1,14 @@
+---
+id: system-arg
+name: System.arg
+category: system
+kind: namespace-method
+tokens: System.arg
+sig: System.arg(i) -> str
+tip: A command-line argument by index.
+order: 0
+ns: System
+member: arg
+---
+
+Returns the <code>i</code>-th command-line argument; index 0 is the program path.