Summary

The synthesized output state collection in debugger/cli/src/main.rs included
outputs without covenant binding, e.g. plain P2PK outputs. For terminal
covenant transitions following the singleton lowering pattern, this caused
next_states to have synthesized non-zero length, breaking
require(next_states.length == 0) at script execution.

This patch filters synthesized output states by output_covenant_id.is_some(),
ensuring only covenant-bound outputs contribute to the synthesized next_states
array passed to the user policy function.

Motivation

Singleton covenant patterns commonly use terminal transitions where the user
policy function returns an empty State[] - releasing locked funds to a P2PK
recipient with no continuing covenant state.

Without this fix, JSON-driven test fixtures for these patterns produce script
execution failures at the length check, even though the fixture correctly
describes a terminal transition with no covenant outputs. The failure
originates earlier in CLI output state synthesis but surfaces at the
length assertion.

Reference: silverscript-lang/tests/covenant_declaration_ast_fixtures/lowers_singleton_sugar_transition_termination_allowed_two_field_state.sil

Testing

• cargo test -p cli-debugger

Added a regression test covering a singleton terminal transition with a plain
P2PK output: the JSON fixture executes the release path and reaches the
terminal length check correctly with next_states.length == 0.

27/27 CLI tests pass (was 26, +1 new regression test).