refactor(core): flatten state transformations

[kitlangton]

What

Flatten three state workflows so synchronous transforms, required lookups, and no-op guards are visible at their call sites. The refactor preserves State finalization order, Form errors, and Session projection behavior.

How

• Inline the single-use State transform wrapper and return the interface directly.
• Rename Form's required cache lookup and implement it with Effect.fromOption.
• Use early guards for missing assistant state and absent retry state.

Scope

Behavior-preserving Core cleanup only; no state lifecycle, form settlement, or Session projection changes.

Testing

• bun run test test/state.test.ts test/form.test.ts test/session-projector.test.ts (28 passed)
• bun typecheck from packages/core
• Push hook: bun turbo typecheck --concurrency=3 (40-package workspace)

[kitlangton]

Consolidated into #43933 so the Session execution and state-workflow cleanup can be reviewed together.