feat(env): FileSystem abstraction

[alexsnaps]

Env Abstraction — Implementation Log

Pluggable filesystem backend replacing hardcoded std::fs / libc I/O.

Steps

• Step 1: Define traits + PosixFileSystem — Create src/env.rs with SequentialFile, RandomAccessFile,
  WritableFile, FileLock, FileSystem traits and PosixFileSystem implementation. Add Options::file_system: Arc<dyn FileSystem> defaulting to PosixFileSystem. No existing code changes beyond the new file and options field.
• Step 2: LogWriter and LogReader — Change LogWriter to wrap Box<dyn WritableFile> (instead of
  BufWriter<File>) and LogReader to wrap Box<dyn SequentialFile> (instead of File). Update all callers
  (WAL creation, MANIFEST creation, WAL replay, repair).
• Step 3: Table reader — Change Table::open to take Arc<dyn RandomAccessFile> instead of File.
  read_block and footer reads go through the trait. Table stores Arc<dyn RandomAccessFile>.
• Step 4: TableBuilder — Change constructor to take Box<dyn WritableFile> instead of File.
• Step 5: TableCache — get_or_open calls fs.open_random_access(path) instead of File::open.
  TableCache stores Arc<dyn FileSystem>.
• Step 6: VersionSet — MANIFEST creation/reading and CURRENT file read/write go through FileSystem.
  VersionSet::create and VersionSet::recover take or store the filesystem handle.
• Step 7: lib.rs + stopgap cleanup — Replace all remaining std::fs calls (~30 sites) in Db::open,
  begin_flush, write_flush, finish_flush, do_compaction, delete_obsolete_files, Db::destroy,
  Db::repair, and acquire_lock. Persistence stores Arc<dyn FileSystem>. libc::flock moves into
  PosixFileSystem::lock_file. Also replace all stopgap PosixFileSystem / writable_from_file /
  random_access_from_file / sequential_from_file calls in production code (marked with TODO) with
  Options::file_system — threading the Arc<dyn FileSystem> from Options through Persistence, TableCache,
  VersionSet, and all free functions that create files. Test code may remain hardcoded to POSIX.
• Step 8: Cleanup — Remove direct libc usage from lib.rs. Verify libc is only used inside
  PosixFileSystem. Run full test suite.