Add SignalR architecture overview

[PureWeen]

Add SignalR architecture overview

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Add an in-area SignalR architecture overview for contributors.

Description

Adds src/SignalR/ARCHITECTURE.md, a reference document for contributors and coding agents working in src/SignalR. It follows the in-area ARCHITECTURE.md pattern established by @javiercn in #69147.

The content is durable SignalR knowledge extracted from the wider #69011 effort and checked against current code and tests on main, so SignalR area owners can review and validate it on its own.

• documents how the SignalR server, transports, protocols, clients, and scaleout layers compose and who owns each
• describes connection establishment, hub handshake, dispatch, lifetime, stateful reconnect, and close/abort/timeout boundaries
• captures cross-client capability differences and architectural compatibility invariants
• maps existing SignalR documentation and the verification (test) boundaries for each subsystem

Documentation only: one new file, no runtime, public API, or other documentation changes.

Validation

• git diff --stat origin/main...HEAD: only src/SignalR/ARCHITECTURE.md (275 insertions).
• git diff --check origin/main...HEAD passed.
• All 35 relative links in the document resolve on disk.
• Markdownlint was not run locally; the repository workflow only lints docs/**/*.md, so it does not cover this in-area document. Runtime tests were not run for this documentation-only change.

Related to #69011; this area extraction does not close the wider effort.

[Copilot]

Copilot review overview

🟡 Changes recommended

Two documentation inaccuracies should be clarified before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review tier: Lite
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​SignalR/​ARCHITECTURE.md — Scope permissive parsing to implementation behavior

What changed in this PR

Adds a SignalR architecture overview covering subsystem composition, lifecycle behavior, protocols, transports, clients, and scaleout.

Changes:

• Documents connection, dispatch, reconnect, and failure flows.
• Describes client capabilities and architectural invariants.
• Maps documentation and verification boundaries.

File	Description
src/​SignalR/​ARCHITECTURE.md	Adds the SignalR architecture and verification overview.

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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 aligns with the implementation, and all relative link targets resolve.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Scope permissive parsing to implementation behavior