Composition.Builder defaults the platform to null and can fail at build time when using(...) is omitted

[shaaravraghu]

Type
Bug

Severity
High

Modules

• spawn-application-composition

Evidence

• spawn-application-composition/src/main/java/build/spawn/application/composition/Composition.java:624-626
• The builder constructor sets this.platform = null.
• Composition.java:775-782 later selects this.platform when no per-composable platform is set and immediately calls platform.launch(...).

Why this matters

• The builder API reads like a normal fluent builder, but there is no guard rail that forces using(...).
• Forgetting one builder step causes a null platform to survive all the way to launch time.

Expected behavior

• Either a sensible default platform should be used, or build() should fail fast with a clear validation error before launch begins.

Actual behavior

• The builder can proceed with a null platform until platform.launch(...) is invoked.
• The failure mode is a null dereference instead of an actionable message.

Suggested reproduction

1. Create Composition.Builder.create().
2. Add a composable but do not call using(...).
3. Call build().
4. Observe the failure caused by a null platform reference.

Suggested fix

• Default to LocalMachine.get() if that remains the intended baseline.
• Otherwise validate in build() that every composable resolves to a non-null platform and throw a descriptive exception.

Suggested tests

• Add a test covering builder usage without using(...).
• Add a validation test asserting the thrown message mentions the missing platform explicitly.

Issue body ready to paste
Composition.Builder initializes platform to null and later uses it directly during build when a composable does not define its own platform. If a caller forgets using(...), the failure is deferred until launch time and surfaces as a null-platform crash instead of a clear validation error.`