docs(runtime-subsystem): document intercept hook, annotations, and cache integration

Updates docs/architecture docs/subsystems/4. Runtime & Middleware Framework.md
to reflect the M1 + M2 + M3 work that landed under TML-2143:

- Updated the runWithMiddleware lifecycle to start with the intercept
  step, with hit-path semantics for skipping beforeExecute / runDriver /
  onRow.
- Added the source field to the AfterExecuteResult shape shown in the
  Middleware API section.
- New 'Intercepting Execution' section documenting RuntimeMiddleware.intercept:
  chain semantics, hit path, row shape, verification ordering, error
  path, the WeakMap-correlated plan-identity invariant (with a forward
  reference to ADR 025), and family-agnostic construction with the cache
  middleware as the canonical example.
- New 'Annotations' subsection covering OperationKind, defineAnnotation,
  ValidAnnotations<K, As>, lane integration on SQL DSL + ORM Collection,
  the runtime applicability check, plan.meta.annotations storage, and
  reserved namespaces (codecs, target keys).
- Documented the new RuntimeMiddlewareContext fields: identityKey(exec)
  (BLAKE2b-512 digest of the execution identity, populated by family
  runtimes) and scope ('runtime' | 'connection' | 'transaction').
- Updated the lifecycle sequence diagram to branch on the intercept hit
  vs. driver path and show source: 'middleware' / 'driver' on
  afterExecute.
- Updated the Execution Pipeline and Connection Lifecycle text snippets
  to reflect the intercept step.
- Updated the Rewriting ASTs scope note: short-circuiting via intercept
  and user-authored annotations are no longer 'deferred to later
  milestones' — they are documented above.
- Extended the Testing Strategy with the intercept matrix, annotation
  surface, family identityKey impls, cache middleware unit + integration
  coverage, and SQL runtime scope plumbing.

Closes C.2 of the cache-middleware project.

Author: aqrln