docs: define post-ASAP physical compilation boundary

[zzylol]

Follow-up: #586
clarifies that ASAPPlanner owns abstract window-framework candidates, while
ASAPQuery-backend owns their concrete runtime implementations and feeds
DataWorkload-specific cost evidence back to Planner.

Why

The existing planner-integration and physical-planning pages describe parts of the boundary, but do not give one canonical, executable contract for turning a deployment-independent Post-ASAP DAG into coordinated runtime plans.

Before this PR

Ownership of sampling, GOS, delta/checkpoint policy, stage partitioning, and error-budget allocation had to be inferred across several documents. The prior draft also treated all plan views like public APIs and did not fully specify in-flight drift, restart replay, SDK activation, or durable frame acknowledgement.

After this PR

One architecture page defines this boundary:

ASAPPlanner Post-ASAP DAG
        -> ASAPQuery-backend physical compiler
        -> SDK startup artifact + CollectorPlan + BackendPlan

TransmissionPlan is a shared typed section; backend precompute and query routing are BackendPlan sections. ASAPPlanner remains deployment-independent, while the backend control plane owns placement, sampling, GOS, delta/full transmission, and runtime adaptation.

The document now also defines:

• error composition in one query-output metric, with explicit failure-probability allocation;
• family-specific threshold domains and a delivery-aware drift bound;
• one-in-flight/one-pending backpressure behavior;
• patched-OTLP FrameAck semantics and atomic backend sequence application;
• Collector WAL and same-epoch restart replay requirements;
• static MVP SDKPlan delivery and semantic activation evidence;
• versioned cross-repository golden artifacts;
• deterministic and statistical SDK sampling acceptance tests; and
• an implementation-status table separating existing partial components from remaining MVP work.

Validation

• git diff --check
• verified balanced display-math delimiters
• verified unsupported operatorname/environment macros are absent
• verified added relative documentation links
• MVP collector CI runs on the pushed commit

Scope

Documentation only. No runtime types, planner adapter, wire schema, or deployment behavior changes in this PR.