fix(pf-vdisplay-proto): offset asserts + own the gamepad SHM layouts (audit §6.1/§6.2)

§6.2: add offset_of! asserts to SharedHeader/AddReply/control structs so a same-size field reorder is a compile error, not silent corruption (size+Pod alone miss it).

§6.1: add XusbShm (64B) + PadShm (256B, incl device_type@140) layouts + Global\ name helpers + magics to the proto crate as the single source of truth, with offset asserts pinned to the shipped wire layout — kills the hand-duplicated literal-140 host/driver drift hazard. Enables bytemuck min_const_generics for the >32-byte reserved tails. Host + driver consumers switch in a follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/pf-vdisplay-proto/Cargo.toml

@@ -16,4 +16,5 @@ description = "Shared host<->driver binary contract for the punktfunk pf-vdispla
 publish = false
 
 [dependencies]
-bytemuck = { version = "1.19", features = ["derive"] }
+# `min_const_generics`: Pod/Zeroable for `[u8; N]` of any N (the gamepad SHM reserved tails are >32).
+bytemuck = { version = "1.19", features = ["derive", "min_const_generics"] }