stdlib: decide & document the namespace/module model (modules vs flat-deduplicated) #132

Part of #128.

The stdlib has no module/import story — zero files use use/import/open, and it relies on a flat global namespace seeded with builtins (see band-aid b895374).

Task (decision + doc): decide and write up the stdlib namespace model:

• (a) real modules with use/qualified paths (Option::Some), or
• (b) intentionally flat + de-duplicated prelude.

Capture the decision in a short ADR / MODULE-SYSTEM-PROGRESS.md section. No code change here — this unblocks the dedup work.

Blocks: #128-C, #128-G.