[browser][coreCLR] R2R: Dedicated JS trigger to capture and download the PGO trace

# Dedicated JS trigger to capture and download the PGO trace

Part of #130524.

Status: **draft / issue candidate** · Area: CoreCLR-on-WASM, diagnostics/EventPipe, developer UX · Related: #130517 (interp instrumentation), #130521 (PGO feature on WASM)

## Goal

A small, developer-invokable JavaScript function — callable from the browser dev-tools console — that captures the in-progress profile as an EventPipe trace and downloads it as a file, mirroring the existing EventPipe developer experience. This lets a profile be collected from a real browser run and converted offline into the precompiler's input.

## Why

The interpreter populates the shared profile data (#130517), and the existing EventPipe event can carry it out, but browser emission is otherwise tied to process shutdown. A developer needs an explicit, in-session way to flush and download the trace without exiting the app. A trivial JS trigger plus a file download is far simpler than standing up a diagnostics server or network transport — the same rationale as the existing EventPipe developer flow.

## Direction

- **Expose a JS-facing function** that starts/stops or flushes the relevant EventPipe session and forces the profile rundown, so the data is emitted mid-session rather than only at shutdown.
- **Deliver the trace as a browser download** — no server side.
- **Feed the existing offline conversion.** The downloaded trace flows into the already-existing conversion to the precompiler's profile input; no new trace format or tool.
- **Developer/opt-in only.** Not part of the default app.

## Scope

**In:** the JS-facing trigger; forcing/flushing the profile emission in-session; downloading the trace file.

**Out:** the interpreter instrumentation (#130517) and PGO feature enablement (#130521); the offline trace-to-profile conversion (existing tooling); any customer-facing productization.

## Success criteria

- From a running browser app, a developer calls the function and receives a trace file that converts into a usable precompiler profile for that run.

## Open questions

- How to force the profile rundown/flush without a process exit (in-session stop semantics).
- Which EventPipe configuration/providers to enable for the capture.
- Capture cost and download mechanics for larger traces in the browser.

> [!NOTE]
> This issue was drafted with GitHub Copilot assistance.