Add Live Debugger package

[watson]

Motivation

Introduce the @datadog/browser-debugger package to enable Live Debugger in browser applications. This gives frontend developers the ability to add log probes to running applications, evaluate conditions, and inspect runtime state — all without redeploying or modifying source code.

Changes

New package: packages/debugger

A new @datadog/browser-debugger package that provides the full probe execution pipeline:

• domain/api.ts — Core instrumentation hooks (onEntry, onReturn, onThrow) that execute probes when instrumented functions are called, including condition evaluation, snapshot capture, template message rendering, and rate limiting.
• domain/probes.ts — Probe lifecycle management (add, remove, clear) with per-probe and global snapshot rate limiting. Compiles probe conditions and template segments on registration.
• domain/capture.ts — Deep value capture for arguments, locals, return values, and thrown errors with configurable reference depth, collection size limits, and string length limits.
• domain/expression.ts — Expression compiler that parses JSON expression trees (comparisons, logical operators, member access, string operations, etc.) into executable functions.
• domain/condition.ts — Probe condition evaluator that compiles and caches condition expressions.
• domain/template.ts — Template segment compiler and evaluator for rendering dynamic probe messages with runtime context.
• domain/stacktrace.ts — Stack trace capture and parsing from Error objects.
• domain/deliveryApi.ts — Polling-based probe delivery client that fetches probe updates/deletions from the Delivery API using a cursor for incremental sync.
• transport/startDebuggerBatch.ts — Transport layer that reuses @datadog/browser-core's batch/flush infrastructure to send debugger snapshots to the logs intake.
• entries/main.ts — Public API surface (datadogLiveDebugger.init(), addProbe(), removeProbe(), clearProbes(), getProbes()). Exposes $dd_entry/$dd_return/$dd_throw hooks on globalThis for instrumented code.

Changes to @datadog/browser-core

• Added 'dd_debugger' as a valid source in configuration and transport types.
• Exported computeTransportConfiguration and the Batch type so the debugger package can create its own transport.

Sandbox / Tooling

• sandbox/debugger/ — Local testing sandbox with an HTML test page, sample probes, and a Remote Config proxy server for end-to-end manual testing against the Delivery API.
• test/apps/instrumentation-overhead/ — Webpack test app for measuring instrumentation overhead.
• test/performance/ — New instrumentation-overhead performance benchmark scenario.
• Updated scripts/build/build-test-apps.ts and scripts/dev-server/lib/server.ts to include the new test app.
• Added debugger entry point to ESLint side-effects allowlist.

Test instructions

1. Unit tests: Run yarn test:unit --spec packages/debugger/src/domain/api.spec.ts (and similarly for capture.spec.ts, condition.spec.ts, deliveryApi.spec.ts, expression.spec.ts, probes.spec.ts, stacktrace.spec.ts, template.spec.ts) — ~2,930 lines of spec coverage across 8 spec files.
2. Sandbox testing: See sandbox/debugger/rc-proxy/QUICKSTART.md for local end-to-end testing with the Delivery API proxy.
3. Performance testing: yarn build:apps && yarn test:performance to run the instrumentation overhead benchmark.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file

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