Add storage service stdlib module in ylang

[Copilot]

Rewrites a C storage service (filesystem/persistent storage in userspace) into idiomatic ylang, following the same syscall-only patterns as fs.y and print.y.

API surface (lib/stdlib/fs/storage_service.y)

• Memory: allocBuffer/freeBuffer — mmap/munmap wrappers
• Path: buildPath(basePath, key) — shared path concatenation helper
• Low-level I/O: storageOpen, storageClose, storageRead, storageWrite, storageSeek, storageFileSize
• Key-value store: storageInit, storagePut, storageGet, storageDelete, storageList, storageRename, storageAppend

The KV store is flat-file backed — each key is a file under a base directory, all ops via Linux x86-64 syscalls with no C runtime dependency.

Example usage

import "stdlib/fs/storage_service";

main() -> {
    storageInit("/tmp/mystore");
    storagePut("/tmp/mystore", "session_id", "abc123", 6);

    let buf = allocBuffer(4096);
    let n = storageGet("/tmp/mystore", "session_id", buf, 4096);
    // buf now contains "abc123", n == 6

    storageList("/tmp/mystore");   // prints "session_id\n" to stdout
    storageDelete("/tmp/mystore", "session_id");
    freeBuffer(buf, 4096);
    return 0;
}

All 16 functions parse successfully against the existing lexer/parser. Existing tests unaffected.