Defer throwing signal work until observation

[dajiaohuang]

Problem

Signal(catching:) evaluates its throwing closure while constructing the signal and captures that single Result. Consequently subscribing twice repeats the first result rather than the work, and retry(1) cannot recover from a transient failure in that closure.

For example, a throwing producer that fails on its first invocation and returns 42 on its second is invoked only once; observing signal.retry(1) receives the cached failure, not 42. This differs from the documented cold-signal behavior and the analogous Signal(performing:) / Signal(evaluating:) initializers.

Change

Defer Result(catching:) through the existing evaluating: initializer. Add tests for deferred/per-observation execution, failure propagation, and retry recovery, including the explicit Linux test registration.

Validation

• Swift 6.0.3, Ubuntu 24.04 (WSL), package's existing Swift language mode: swift build -j 2 passed.
• New regressions against unchanged production code: 3 tests executed; the deferred/per-observation and retry cases failed (6 assertions), failure propagation passed.
• After the fix: swift test -j 2 --filter 'SignalTests.test(Catching|ProductionAndObservation|Just|Sequence|Completed|Failed|Map|Filter|Retry$|Append$)': 12 tests passed.
• Separate tiny local consumer package using normal integer values: 9 finite tests passed, covering cold operators, disposal, subjects/replay, properties/bindings, loading/retry, a bounded serial scheduler, Published, async bridges, and this throwing-factory regression.
• git diff --check passed.

Apple runtimes/Xcode, Android, and the full upstream suite (including stress tests) were not run. Existing Swift 6 concurrency warnings remain; no Swift 6 language-mode or all-platform acceptance claim.