feat(gateway): Phase B — Result shaping on invoke

Autonomous decisions:
- DEFAULT_MAX_ROWS=50 and DEFAULT_MAX_BYTES=65536 — balances context savings with usable samples for smart default truncation
- pub mod invoke — external integration tests need public access to shaping helpers
- Hoist returned/total/truncated to parent object for nested list keys — keeps truncation metadata visible without extra nesting

Signed-off-by: crimsonsunset <jsangio1@gmail.com>

Author: crimsonsunset