Add Kestrel architecture overview

[PureWeen]

Summary

• add an area-owner-reviewable architecture overview for Kestrel
• document startup and Hosting handoff, endpoint binding, transports, TLS and protocol selection, HTTP/1.1, HTTP/2, and HTTP/3 lifetimes, request features, body I/O and backpressure, timeouts, shutdown, pooling, diagnostics, generation, and verification boundaries
• define adjacent ownership explicitly so Kestrel does not claim HttpSys, IIS/ANCM, generic HTTP abstractions or middleware, Hosting request lifetime, higher-level SignalR/gRPC semantics, runtime socket/TLS/QUIC internals, or reverse-proxy/application policy

Context

#69011 is the wider servers/networking review-guidance effort. This PR extracts the durable Kestrel-owned technical knowledge into a focused architecture document for server-owner validation.

The single-file shape follows @javiercn's merged Components architecture document in #69147 and the sibling architecture work in #69200, #69216, #69242, #69243, and #69292.

Validation

• effective diff is exactly one additive file: src/Servers/Kestrel/ARCHITECTURE.md
• git diff --check origin/main...HEAD
• all repository-relative links resolve
• generated Markdown anchors are unique
• all referenced external documentation URLs return successful responses
• the existing repository markdownlint workflow targets docs/**/*.md, so it does not apply to this source-area document

[Copilot]

Copilot review overview

🟡 Changes recommended

The architecture document has unresolved accuracy and verification-guidance issues.

Get a fresh assessment by requesting another Copilot review.

Review tier: Lite
Findings: 3

Open (3)

• Describe HTTPS initialization rather than service registration · New
• Do not claim body limits run before request features · New
• Point transport verification links at the actual test projects · New

What changed in this PR

Adds a Kestrel architecture overview covering ownership, startup, transports, protocols, I/O, shutdown, diagnostics, and verification.

Changes:

• Documents Kestrel subsystem composition and protocol lifetimes.
• Defines ownership boundaries and verification locations.
• Covers HTTP/1.1, HTTP/2, HTTP/3, TLS, pooling, and backpressure.

File	Description
src/​Servers/​Kestrel/​ARCHITECTURE.md	Adds the Kestrel architecture overview.

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