Add spec 023: performance insight tools

[codemonkeychris]

Summary

• Proposes spec 023: a WinUI-first set of performance insight capabilities — compile-time PerfReport.json, default-on framework signposts, stable per-element source IDs (PerfTag), a public dependency-graph query API, and an agent-consumable JSON trace format.
• Reactor contributes a narrow set of component-layer extensions (Component Intent kind, component-tree rollups, reconciler-aware metrics) on top of WinUI capabilities rather than owning a parallel stack.
• Includes a competitive teardown of Apple (Instruments, os_signpost, SwiftUI instrument, MetricKit, Apple Silicon PMCs) and Google (Perfetto, Compose Compiler Metrics, Baseline Profiles), with a precise table of where Windows/ETW has parity vs. where we lack convention layers.
• Lays out three attribution-mechanism options (AsyncLocal / Correlation Vector / sampling) with pros/cons and a non-binding recommendation.
• Proposes System.Diagnostics.Signpost in the .NET BCL as the single highest-leverage API (noted as possibly a separate spec).

Test plan

• Review problem framing and confirm WinUI-first scoping matches intent
• Validate the WinUI vs Reactor responsibility split table (§8) — confirm every row lands in the right org
• Sanity-check the Microsoft.UI.Xaml.Controls.Perf claims against the actual provider in microsoft/microsoft-ui-xaml
• Verify references to existing specs (007, 022) are accurate
• Confirm the open questions (§16) capture all ambiguous decisions rather than pre-committing them

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