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/mem/mem-fill.md

@@ -1,14 +0,0 @@
----
-id: mem-fill
-name: Mem.fill
-category: mem
-kind: namespace-method
-tokens: Mem.fill
-sig: Mem.fill(buf, value, n) -> void
-tip: Set n bytes to a value.
-order: 3
-ns: Mem
-member: fill
----
-
-Sets the first <code>n</code> bytes of <code>buf</code> to the byte <code>value</code>.