Honor |=> captures on composite THEN / IF / LOOP nodes

[Copilot]

|=> / df.as() could bind to a composite node because PostgreSQL operators are left-associative, but composite executors were not writing their final value into the named-results map. That made downstream $name references fail at runtime even though the capture appeared to succeed.

• Runtime behavior

  • Add shared named-result storage for composite executors.
  • Make THEN store the right-hand result when result_name is set.
  • Make IF store the selected branch result when result_name is set.
  • Make LOOP store its terminal result, including df.break(...) values, when result_name is set.

• Regression coverage

  • Add an E2E case for the reported signal flow where |=> 'decision' binds to the outer THEN node and the captured value is referenced in a downstream df.if(...).
  • Add E2E coverage for naming an IF node directly and using $decision.* downstream.
  • Add E2E coverage for naming a LOOP node directly and consuming the loop’s final JSON result downstream.

• Result

  • Composite nodes now behave like atomic nodes with respect to |=>: the final computed result is available for later substitution under the requested name.

Example:

SELECT df.start(
    'INSERT INTO test_log (msg) VALUES (''Waiting...'')'
    ~> df.wait_for_signal('test_approval', 60) |=> 'decision'
    ~> df.if(
        'SELECT ($decision::jsonb->''data''->>''approved'')::boolean',
        'INSERT INTO test_log (msg) VALUES (''APPROVED!'')',
        'INSERT INTO test_log (msg) VALUES (''REJECTED!'')'
    ),
    'signal-var-fix'
);

[pinodeca]

Reviewed with GPT-5.5 in GitHub Copilot.

No blocking findings. The composite-node capture change looks consistent with existing named-result handling for JOIN/RACE/HTTP/SIGNAL, and the rebase conflict in 07_signals.sql now keeps both main's plain-text signal test and this PR's composite THEN capture test.

Validated locally: cargo fmt -p pg_durable -- --check, ./scripts/test-e2e-local.sh 02_conditionals, ./scripts/test-e2e-local.sh 03_loops, and ./scripts/test-e2e-local.sh 07_signals passed. cargo clippy --features pg17 -- -D warnings was blocked by an unrelated pre-existing unused extract_host warning in src/ssrf.rs.