[Gulliby]

Requested by gulliby

Add a --struct NAME option to structpad layout.

Before: layout always printed every struct in the file, which is long when only one record matters, and a nested struct such as Frame cannot be cut out of the file because it needs Packet and Wire defined before it.

After: structpad layout --struct Frame --struct Wire net.sp prints only those structs, once each and in file order, with the same numbers as the full listing (nested structs are still resolved from the whole file). --struct=NAME also works, the option may appear anywhere and be repeated, and without it nothing changes.

struct Wire: size 5, align 1, padding 0, packed
offset   size  field
     0      1  kind: u8
     1      4  value: u32

struct Frame: size 56, align 8, padding 12
offset   size  field
     0      1  version: u8
     1      7  (padding)
     8     32  head: struct Packet
    40     10  tail: struct Wire[2]
    50      1  ok: bool
    51      5  (padding)

Errors exit with status 2 and one stderr line: invalid --struct "X", --struct needs a value, suggest does not take --struct, check does not take --struct, unknown struct "X" in --struct.

How: src/structpad/cli.py (option parsing, validation, usage line) and src/structpad/render.py (render_layout takes an optional selection), README section, the usage constant in tests/test_cli.py, and 13 new tests in tests/test_struct_select.py (131 in total). No dependency or CI changes.