Add Kestrel architecture overview

[PureWeen]

Add Kestrel architecture overview

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Add an in-area Kestrel architecture reference.

Description

Summary

• add src/Servers/Kestrel/ARCHITECTURE.md as a reference for contributors and coding agents only
• 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

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

This is a documentation-only addition. It changes no production code, public APIs, dependency manifests, or build configuration.

Validation

• effective diff is exactly one new file: src/Servers/Kestrel/ARCHITECTURE.md
• git diff --check origin/main...HEAD: passed
• all repository-relative links resolve
• generated Markdown anchors are unique
• markdownlint was not run locally

Related to #69011.

[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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[dotnet-policy-service]

Looks like this PR hasn't been active for some time and the codebase could have been changed in the meantime.
To make sure no conflicting changes have occurred, please rerun validation before merging. You can do this by leaving an /azp run comment here (requires commit rights), or by simply closing and reopening.

[Copilot]

Copilot review overview

🟢 Approval recommended

The documentation accurately reflects the reviewed implementation, and all repository-relative links resolve.

Review effort: Balanced
Findings: None

Resolved since last review (3)

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