[demo split #4449] 1/5 — Foundation: expose core hooks for the live debugger package

[allspain]

DEMO — split of #4449. This is one of 5 stacked draft PRs that breaks
#4449 — Add Live Debugger package
into reviewable increments. Original PR is untouched. This stack is for
demonstrating tooling (jj split for the surgery, gh for the stack);
all of the underlying code is Thomas Watson's work and credited via
Co-authored-by: trailers.

Stack

1. [demo split #4449] 1/5 — Foundation: expose core hooks for the live debugger package #4532 — Foundation — expose core hooks for the live debugger package (12 files / +56-10) ← you are here
2. [demo split #4449] 2/5 — Add @datadog/browser-debugger package #4533 — Package — add @datadog/browser-debugger (25 files / +6433)
3. [demo split #4449] 3/5 — Extend e2e framework for the debugger package #4534 — E2E framework — extend test framework for the debugger package (12 files / +129-8)
4. [demo split #4449] 4/5 — Add debugger e2e scenarios #4535 — E2E scenarios — add debugger e2e scenarios (3 files / +318-2)
5. [demo split #4449] 5/5 — Add instrumentation overhead benchmarks #4536 — Perf benchmarks — add instrumentation overhead benchmarks (12 files / +1222-9)

Each PR's base branch is the previous PR's head, so reviewing in order is
the intended path. Stack constructed with jj:
all 4 commits from #4449 were squashed into one change, then peeled into 5
with jj split <paths>, then pushed with jj git push.

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What this PR contains (1/5 — Foundation)

Pre-requisite changes to @datadog/browser-core and project-level config that
the rest of the stack depends on. Intentionally small — this is the
foundation that everything builds on.

Changes

• packages/core
  • domain/configuration/configuration.ts, endpointBuilder.ts,
    transportConfiguration.ts — add 'dd_debugger' as a valid transport
    source, mapped to 'browser'.
  • src/index.ts, transport/index.ts — export Batch type and
    computeTransportConfiguration so the debugger package can build its
    own transport.
• Tooling
  • eslint-local-rules/disallowSideEffects.js — allow the debugger entry
    point to register globalThis hooks.
  • tsconfig.base.json — register the new package path.
  • typedoc.json — exclude the package from generated docs (per-spec, since
    this is internal-only until validated in production).
  • .github/CODEOWNERS, LICENSE-3rdparty.csv, test/unit/browsers.conf.ts,
    yarn.lock — wiring updates.

12 files, +56 / -10.

Why this stack

PR #4449 lands 64 files in a single ~8.2k-line PR, which is hard to review
end-to-end. Splitting into reviewable increments (this stack) keeps each PR
focused on a single concern and makes regressions easier to bisect.

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