feat: add Environment launch snapshot API

[vycdev2]

Summary

• add source-owned Environment.get and Environment.args over private trusted runtime kernels
• freeze user arguments and environment entries once per runtime context, with deterministic isolated snapshot injection for tests
• remove ambient os.env / os.args, add focused migration diagnostics, capability restrictions, and contract documentation updates

Verification

• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed
• cargo check -q --target x86_64-pc-windows-gnu — passed
• cargo run -q -p jett_cli -- format --check stdlib/environment.jett — passed
• cargo run -q -p jett_cli -- format --check tests/run_pass/environment_snapshot.jett — passed
• git diff --check — passed

Risk

• Medium: this changes the interpreter launch-data boundary and removes legacy ambient builtins; deterministic provider, source-signature, migration, purity, namespace, Unicode, duplicate-entry, and argument-list coverage is included.

Closes #170

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