refactor(stdio): remove fd table, forward opens to primary backend

Remove the internal fd table (fd_entry_t, is_fd_valid, fd_allows_read/write,
ESP_STDIO_BASIC_MODE tracking) from the console VFS.

Instead, console_open forwards directly to the primary backend's open_p,
so each open("/dev/console") creates a real fd in the underlying driver.
Read/write/fstat/fcntl/fsync all operate on the per-open fd from the
primary, not a stored init-time fd.

Structure:
- s_primary: separate variable for the primary backend entry
- s_auxiliary[]: slot-based array for auxiliary sinks (NULL = free)
- Scans all slots (no count variable) with NULL guards
- Auxiliaries opened lazily on first console open
- Closed when last console fd is closed (refcounted via s_open_count)
- Write and fsync fan-out protected by s_lock

components/esp_stdio/Kconfig

@@ -185,28 +185,6 @@ menu "ESP-STDIO"
             If enabled, esp_rom_printf and ESP_EARLY_LOG output will also be sent over USB CDC.
             Disabling this option saves about 1kB or RAM.
 
-    config ESP_STDIO_BASIC_MODE
-        bool "Use basic /dev/console descriptor handling"
-        depends on VFS_SUPPORT_IO
-        default n
-        help
-            Enable the low-overhead legacy /dev/console descriptor handling.
-
-            In basic mode, all open() calls share a single internal descriptor. This keeps RAM usage
-            lower, but disables checks for per-fd behaviour, such as read-only versus write-only access.
-
-    config ESP_STDIO_MAX_FDS
-        int "Max logical stdio file descriptors"
-        depends on VFS_SUPPORT_IO && !ESP_STDIO_BASIC_MODE
-        range 3 64
-        default 16
-        help
-            Maximum number of logical /dev/console descriptors tracked by esp_stdio VFS.
-
-            stdin, stdout, and stderr each occupy one slot. The default leaves room for additional
-            opens of /dev/console (e.g. dup2, reopen). If set to exactly 3, only the three standard
-            streams fit and further opens fail with ENFILE unless one is closed first.
-
     config ESP_STDIO_MAX_VFS_ENTRIES
         int "Maximum number of registerable VFS sinks"
         depends on VFS_SUPPORT_IO