SDP_DeveloperGuide.adoc: add 'Copy Up Scope: En-Masse vs.
Selective' section, documenting the two common models and their tradeoffs, and recording the decision that SDP uses En-Masse (Copy Up Everything) -- ties together with dev's always-release-ready requirement and the Release Process's all-or-nothing Copy Up step.
Prompted by the user expanding on yesterday's Copy Up cadence correction. Deliberately staying in dev only, per that same policy.