Move simple C shim outputs to MoonBit allocation

[peter-jerry-ye]

Part of #408. Stacked on #413.

Why

Several small C shims allocate returned String, Bytes, or arrays directly.
For Wasm1 reuse, MoonBit should own those visible allocations and C should only
fill caller-provided memory.

What

• Move temporary path allocation and retry logic into MoonBit.
• Move Unix and Windows current-environment allocation into MoonBit.
• Move the dump-env helper to caller-provided output buffers.
• Move Windows OS-string decode output allocation into MoonBit.

Why this is correct

Each shim now returns the actual required length. C copies into the
MoonBit-provided buffer only when it fits; otherwise MoonBit allocates the
reported size and retries. The Windows environment block keeps its full
double-null-terminated representation.

Review scope

This PR is limited to simple synchronous fill-return shims and test helper
bindings. TLS and thread-pool jobs are split into later PRs.

[coveralls]

Coverage Report for CI Build 458

Warning

No base build found for commit 0808928 on codex/wasm1-socket-ifname-buffer.
Coverage changes can't be calculated without a base build.
If a base build is processing, this comment will update automatically when it completes.

Coverage: 78.894%

Details

• Patch coverage: 20 uncovered changes across 4 files (22 of 42 lines covered, 52.38%).

Uncovered Changes

File	Changed	Covered	%
src/fs/tmpdir.mbt	12	4	33.33%
src/process/windows.mbt	10	4	40.0%
src/internal/os_string/os_string.mbt	12	8	66.67%
src/process/unix.mbt	8	6	75.0%

Coverage Regressions

Requires a base build to compare against. How to fix this →

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Coverage Stats

Relevant Lines:	4269
Covered Lines:	3368
Line Coverage:	78.89%
Coverage Strength:	78902.64 hits per line

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💛 - Coveralls

[Guest0x0]

close via #415. Note that:

• everything under test_programs don't need to support WASM, so no need to migrate them
• @os_string would probably need separated #cfg for wasm anyway, so no need to migrate its current native code