Fix random_activation_in_scope panic and simplify implementation

[Copilot]

ActivationRegistry::random_activation_in_scope would panic via usize underflow when the registry had 0 entries (len() - 1 wraps), and produce an empty range panic with exactly 1 entry. The cyclic-iterator approach was also unnecessarily convoluted.

Changes

• src/activation.rs: Replace the cyclic-iterator loop with a direct call to activations_in_scope, assert non-empty with a descriptive message, then index randomly:

// Before
let mut iter = self.fns.values().cycle();
let num_iterations = rng.random_range(0..self.fns.len() - 1); // panics on len 0 or 1
for _ in 0..num_iterations { iter.next().unwrap(); }
let mut val = iter.next().unwrap();
while !val.scope.contains(scope) { val = iter.next().unwrap(); }
val.clone()

// After
let activations = self.activations_in_scope(scope);
assert!(!activations.is_empty(), "no activation functions registered for scope {:?}", scope);
activations[rng.random_range(0..activations.len())].clone()

• Added # Panics doc section documenting the empty-scope assertion.

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