Honor append and truncate flags in native Windows file opening

Two branches in Windows `open` do not preserve the advertised file flags:

- `FS_O_TRUNC` only influences creation disposition when `FS_O_CREAT` is also present. Opening an existing file with WRONLY | TRUNC selects OPEN_EXISTING and leaves its contents intact.
- APPEND adds FILE_APPEND_DATA to GENERIC_WRITE and seeks to EOF once at open time. GENERIC_WRITE still allows positional writes. A later seek can make a write overwrite existing content, and independently opened append handles can retain the same old EOF position.

These belong in one review of native open-flag translation, rather than separate reports for every flag combination.

Source evidence at the head of #520:

- [std/sys/windows/fs.wave:104](https://github.com/wavefnd/Wave/blob/2ae0a91c57fd76323124c0e4dff552adbfadbe1b/std/sys/windows/fs.wave#L104) — `pub fun open`

Acceptance:

- [ ] Implement truncation of an existing writable file without requiring CREAT, while a missing file still fails without CREAT.
- [ ] Make each append write target EOF using native append semantics; a one-time seek is insufficient.
- [ ] Cover append after an explicit seek, two independently opened append handles, TRUNC without CREAT, and CREAT|EXCL on an existing file.
- [ ] Check native call failures and preserve existing data when opening fails.

API/reference context: [CreateFile creation dispositions](https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea) and [file access rights](https://learn.microsoft.com/en-us/windows/win32/fileio/file-security-and-access-rights).

Related: #385.

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.

Start at pub fun open in std/sys/windows/fs.wave:104 and compare its flag translation with the linked CreateFile creation dispositions and access-rights references. Exercise the listed truncation, append, exclusive-create, seek, and independent-handle scenarios; done means native failures are checked and failed opens preserve existing data.

Written by the indexing model from the issue text.