fix: add bounds checking to prevent unbounded vector growth from plugins

[sneurlax]

This PR:

1. Adds the bounds constant MAX_STANZAS_PER_PHASE = 1024 to limit the number of stanzas per phase.
2. Modified the validate function to return io::Result<()> and check bounds before adding to vectors.
3. Returns an error if the number of stanzas exceeds the limit.
4. Updated all validate calls to propagate errors using ? while also preventing unbounded error vector growth.

Closes #578