docs: add developer guides for backend design components

[zzylol]

Summary

Follow up #445 with developer guides for the public component interfaces required by the active designs.

Every developer guide now follows the same required structure:

1. Code architecture — public module/component boundaries and data flow.
2. Public interfaces and definitions — public traits, structs, enums, field semantics, function signatures, inputs, outputs, errors, and why the interfaces exist.
3. Adding and verifying functionality — how to add concrete feature classes through those interfaces and how to interpret/validate their outputs.

The guides do not document private interfaces, functions, variables, implementation files, or internal helpers. Where current code has not converged on a stable public boundary, the document explicitly labels a target public API rather than presenting migration internals as API.

Public interfaces covered

Control plane

• Planner adapter: PlanningRequest -> SelectedLogicalPlan.
• Physical compiler: selected plan + deployment/policy -> matching CompiledPlanBundle.
• Runtime clients: CollectorPlan and BackendPlan staging.
• Semantic application reports and PlanPublisher activation/rollback.

Data plane

• BackendPlan decoder, validator, runtime, snapshots, and lifecycle reports.
• OTLP SummaryDecoder, SeriesIdentityResolver, SummaryValidator, and SummaryStateApplier.
• QueryService, SummaryReader, and ExactQueryClient.
• SeriesRegistry and SummaryStore, including typed coverage results.
• Public protocol server/adapter/fallback extension contracts.
• Cross-repository SummaryCapability workflow for adding a summary family.

ASAPCollector correspondence

Runtime publication directly references ASAPCollector's authoritative docs/developer_docs/opamp-config-push.md rather than redefining CollectorPlan. It preserves:

• the asap-collector-plan.yaml AgentConfigMap entry;
• YAML media type;
• config_hash versus cross-runtime plan_id;
• io.asap.collector.plan.v1 / application_report semantics; and
• the documented current implementation gap.

Navigation

• control-plane and data-plane indexes list the new guides;
• each active design links to its corresponding developer interface;
• repository documentation index links to all public extension guides.

Validation

• git diff --check
• all local Markdown links resolve;
• all developer documents have the required three sections;
• fenced code blocks are balanced;
• checked that developer docs contain no pub(crate), private interface/function/variable documentation, implementation file paths, or current-code maps;
• documentation-only change.