docs: track OS Environment contract

[vycdev2]

Summary

• link the Phase K os status to the Environment/argv contract in design: define os Environment and argv contract #94
• surface the same tracker beside Rule Set 16 and the architecture capability/stdlib inventories

Verification

• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed (full workspace suite, including all 432 driver fixtures)
• git diff --check — passed

Risk

• Low: documentation-only tracker backlinks; no language or runtime behavior changes.

Tracks #94

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[vycdev]

The OS Environment tracker backlinks reviewed cleanly, but this branch now conflicts with the documentation already landed on current main. Please rebase, preserve the current architecture/design/progress entries, and rerun diff/link checks.