Renames the Swap instruction and its guest handler to SwapExactInput to
distinguish it from the newly added SwapExactOutput, and to make the
intent of each variant explicit at the call site.
BREAKING CHANGE: the Swap instruction variant and swap() function are
renamed to SwapExactInput and swap_exact_input(). Callers must update
instruction construction and any IDL-generated bindings.
Adds SwapExactOutput to the AMM, allowing callers to specify the exact
desired output amount while the protocol computes the required input
(ceiling division to prevent rounding in the protocol's favour).
The swap-exact-output success tests now use a dedicated small-pool
fixture (reserve_a=1_000, reserve_b=500) rather than the shared
pool_definition_init, which had its reserves bumped to 5_000/2_500 in a
later commit to satisfy the MINIMUM_LIQUIDITY invariant introduced for
new_definition. Using a dedicated fixture keeps each test self-contained
and avoids hardcoded expected values silently breaking when shared
baselines change.
