Convert the Windows environment block to UTF-8 instead of copying narrow API bytes

[LunaStev]

env_read copies bytes from GetEnvironmentStringsA into the environment representation consumed by Wave. The narrow block is not guaranteed to contain UTF-8, and characters unavailable in that encoding may already be lost before copying. Unicode environment keys and values therefore need a wide-API conversion path independent of filesystem path handling.

Source evidence at the head of #520:

• std/sys/windows/env.wave:9 — GetEnvironmentStringsA
• std/sys/windows/env.wave:12 — pub fun env_read

Acceptance:

• Read GetEnvironmentStringsW and convert complete entries to UTF-8 while preserving the double-NUL-terminated block format.
• Free the matching native block on success and every failure path.
• Test Korean and supplementary-plane keys/values, an empty value, and destination capacities around the encoded byte length.
• Preserve the environment block rules for special entries and add no libc dependency.

API/reference context: GetEnvironmentStrings.

Related: #385 and #427; #427 requests portable boundary tests, whereas this issue fixes the native encoding implementation.

Audit status: identified by static source inspection; the scenarios above have not been executed during this audit. The permalink fixes the reviewed revision; this report does not claim the defect was introduced by #520.