Fix Windows drive environment overrides

[imakris]

Summary

• parse Windows drive current-directory pseudo variables (=C:=...) by their drive key instead of collapsing every leading-= entry to an empty key
• sort custom Unicode environment blocks with locale-independent, case-insensitive ordinal ordering before CreateProcessW
• cover multiple drives, changed/stale same-key entries, ordinary variables, exact block entries, termination, and ordering
• document Windows drive current-directory override support

Red-to-green evidence

Before the utility source change, the new MSVC Debug regression test built and failed with:

utility_test: env_key_of should preserve a Windows drive pseudo-variable name

After the fix, the directly affected tests pass on:

• MSVC Debug and Release: utility_test, spawn_wait_test (spawn_detached_test is a passing POSIX-only skip on Windows)
• GCC Debug and Release: utility_test, spawn_detached_test, spawn_wait_test
• reference documentation generator

Additional broad observations

Optional Windows all-active builds completed for Debug and Release. Each concurrent once-through ran 130 invocations and produced 129 passes plus one unrelated managed-child timing/setup miss. The Debug crash-provenance contract then passed 4 consecutive focused runs; the Release readiness-deadline contract passed 3 consecutive focused runs. Both environment-related tests passed in the broad runs. These observations are reported for transparency and are not part of the causal gate for this focused environment fix.