Constant MAX_ON_SET_CALLBACKS 

pub const MAX_ON_SET_CALLBACKS: usize = 4; // 4usize

How many on-set-parameters callbacks ONE store holds, across every node.

phase-417 W4.a — a BOUNDED registry with a stated capacity, because the unbounded upstream shape needs an allocator this crate does not have. Four, and the number is chosen rather than inherited: a slot costs size_of::<Option<OnSetCallbackSlot>>() — 32 bytes on a 64-bit host, 16 on a 32-bit target — so the whole registry is 128 bytes next to a 285,184-byte default table, and four is one hook for each node of a four-node composed image (RFC-0047’s shape) with none left to spare by accident. Registering a fifth REFUSES with None rather than dropping one silently.

It is deliberately NOT a build knob. Every other parameter capacity sizes the table a user’s data lands in; this one sizes a table of code the image itself registers, which is known at build time by reading the source. A knob here would be a number nobody can derive and everybody has to state.