fix: os module value access generates undefined 'os' identifier

[noahgift]

Problem

When Python code accesses os module functions/attributes, the transpiled Rust generates references to an undefined os value:

import os
path = os.path.join("a", "b")  # E0425: cannot find value `os` in this scope
cwd = os.getcwd()              # E0425: cannot find value `os` in this scope

Impact

16 files in the reprorusted-python-cli corpus fail due to this issue.

Suggested Fix

Map common os module functions to Rust equivalents:

Python	Rust
os.getcwd()	std::env::current_dir()
os.path.join(a, b)	std::path::Path::new(a).join(b)
os.path.exists(p)	std::path::Path::new(p).exists()
os.path.dirname(p)	std::path::Path::new(p).parent()
os.path.basename(p)	std::path::Path::new(p).file_name()
os.environ.get(k)	std::env::var(k)
os.listdir(p)	std::fs::read_dir(p)

Corpus Evidence

From reprorusted-python-cli error analysis:

• E0425 (cannot find value) accounts for 10.0% of all compilation errors
• 16 examples specifically fail on os module access

Related

• Part of single-shot compile rate improvement effort (Track Single-Shot Compile Rate (Transpile → cargo check) #193)
• Release gate: 80% single-shot (RELEASE GATE: 80% Single-Shot Compile Rate #194)
• Previously identified in roadmap feat: Map os module to std::env/std::fs #196