Stabilize `GenerateSdkLayout` producers

Part of #48070. Audit the producers aggregated by `GenerateSdkLayout`, preserve existing incremental mechanisms, stop content-stable rewrites, and add target-level skipping where inputs and outputs can be modeled completely. Split implementation by producer family rather than attempting one large PR.

**Dependencies:** None. This work can proceed in parallel with the shared installer-layout ownership issues; reuse #55867 where shared fingerprint or output-inventory primitives are needed.

Start by tracing the producers aggregated by GenerateSdkLayout and read the context in #48070. Review #55867 for shared fingerprint or output-inventory primitives; done means producer families are handled separately, existing incremental mechanisms are preserved, content-stable rewrites stop, and eligible targets can be skipped.

Written by the indexing model from the issue text.