Fix truncation when overwriting in-memory files

[rksharma-owg]

Opening an existing MemoryFileSystem file with w or wb currently retains its old buffer contents. Writing "hello", reopening with w, and writing "x" yields "xello"; opening with w without writing leaves the entire old file intact.

Reset the file position and truncate the buffer on write-mode open, updating modification time while preserving creation time. Tests cover shorter and empty replacements for both text and binary files.

This is separate from #429, which addresses path-prefix stripping.

Validation on macOS, Python 3.12.13:

• Both new regression cases fail before the fix.
• I/O tests: 32 passed, 2 subtests passed.
• Full suite excluding the separately run reloader tests: 1720 passed, 1 skipped, 2 subtests passed.
• Reloader tests: 4 passed.
• git diff --check passes.

AI assistance: Codex assisted with investigation, implementation, and local verification.