Add a resolve middleware example

[tillig]

Part of #32, and the Examples half of autofac/Documentation#160.

Proposed Changes

MiddlewarePipelineExample, covering five things resolve middleware is actually good for. Each middleware is its own class; Program runs them in turn and prints what each one did.

Scenario	Middleware	Point
Every registration at once	ActivationAuditMiddleware	The autofac/Autofac#1337 request: a hook for all registered components, with no per-registration call
Timing	TimingMiddleware	Measure activation cost for a service
Short-circuit	CachingMiddleware	Set Instance, never call next, and activation is skipped
Ambient state	CorrelationIdMiddleware	ChangeParameters supplies a value no caller passed
Fail fast	RootScopeGuardMiddleware	Readable error when something is resolved from the root scope

autofac/Documentation#160 asked for real-world middleware examples because the only sample was the Hello World logging lambda. The documentation section that links to this lands in a separate Documentation PR.

The scenario worth reading twice

CorrelationIdMiddleware does not work the obvious way, and that is why it is in here. Parameter selection is a registration pipeline phase, so registering it with RegisterServiceMiddleware throws:

Cannot add registration middleware in phase 'ParameterSelection' to a service pipeline.
Valid registration middleware phases: [ResolveRequestStart, ScopeSelection, Decoration, Sharing, ServicePipelineEnd]

I hit that while writing this and kept it rather than quietly picking a phase that worked. It is attached with ConfigurePipeline instead, and both the code comment and the README say why — that error message is the clearest explanation of why Autofac has two pipelines instead of one, which is the part of the docs page people skim.

Verification

All 20 projects build with 0 errors and 0 warnings in Release, both gates pass, and the example was run rather than only compiled. Every scenario produces its intended output: two components audited without either registration naming the middleware, an 80ms resolve reported, a cache hit that skips activation, two different correlation IDs injected across two resolves without a caller passing either, and the root-scope resolve rejected while the child-scope resolve succeeds.

The validators from earlier PRs all still pass: 12 launch configurations for 12 launchable projects with nothing missing or orphaned, every src directory has a README, and all 99 markdown links resolve.