docs: add backend design and usage guides

[zzylol]

Why

ASAPQuery-backend contained shared system designs that duplicated the canonical documentation and maintained overlapping README.md/design.md files for the same component scope.

What

• Moves shared backend and control-plane designs to ASAPCollector/docs instead of retaining a second copy.
• Removes duplicate Query design files after their content is transferred.
• Consolidates each implementation scope into one main file: summary-store implementation.md, query query-engine.md, ingest ingest-engine.md, and SID resolver.md.
• Retains distinct implementation subcomponents where their scopes differ, such as BackendPlan installation, protocol extension points, physical compilation, and plan publication.
• Updates architecture, user-guide, and developer links to the canonical Collector pages.
• Clarifies that materialization identity describes maintained summary semantics, while SID identifies one canonical materialized series under that definition.
• Expands the physical compiler with concrete CollectorPlan and BackendPlan contents, their consuming Collector/backend components, and the relationship between materialization descriptors and runtime SIDs.
• Expands publication with target-specific artifacts, stage/activate/abort operations, backend-first readiness, correlated reports, activation barriers, drain, and rollback behavior.

Ownership

This repository now documents Rust implementation only. Shared SID semantics, storage behavior, query/ingest boundaries, physical-plan contracts, workload inputs, feedback loops, and future storage design are authoritative in ASAPCollector.

Verification

• git diff --check
• All repository-local Markdown links under docs/ resolve.
• Transferred design content was preserved in ASAPCollector before duplicate Query files were removed.
• Documentation-only change; runtime tests are not applicable.