Loader stub silently drops calls outside queued method list, which results in either silent no-ops

### Problem Statement

## Problem

The CDN loader stub only queues a fixed set of SDK methods pre-load (`init`, `addBreadcrumb`, `captureMessage`, `captureException`, `captureEvent`, `configureScope`, `withScope`, `showReportDialog`). Any SDK call outside this list — such as `addIntegration` — is silently absent on the `window.Sentry` proxy before the full bundle loads, causing either silent no-ops or runtime crashes depending on timing.

The correct workaround is wrapping such calls in `Sentry.onLoad(...)`, but the loader provides no mechanism to enforce this, making it easy to write code that works in development (where the bundle may already be loaded) but crashes in production.

## Current Behavior

- Unrecognized calls on the pre-load proxy are `undefined` — calling them throws or silently does nothing
- `addIntegration` is a notable gap: it is a common post-init call pattern that the loader stub does not queue

## Gap

- No TypeScript or runtime guardrail prevents calling unqueued methods before `onLoad`
- The failure mode is timing-dependent and hard to reproduce locally

### Solution Brainstorm

## Options

1. **Extend the queued method list** — add `addIntegration` (and other commonly used post-init methods) to the pre-load queue so they are replayed after the bundle loads, consistent with how `init` is handled
2. **Catch-all no-op proxy** — intercept any unrecognized property access pre-load and return a no-op function; prevents crashes but silently swallows calls
3. **Catch-all replay proxy** — queue all unrecognized calls (not just the fixed list) and attempt replay post-load; broader coverage but requires more careful handling of methods with return values

Option 1 is the lowest-risk and most targeted fix. Option 3 is the most ergonomic for callers but adds complexity to the replay logic.

### Additional Context

## References

- Loader stub source: `packages/browser-integration-tests/fixtures/loader.js`
- `addIntegration` implementation: `packages/core/src/integration.ts`

### Priority

_{React with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding `+1` or `me too`, to help us triage it.}

Start by reading packages/browser-integration-tests/fixtures/loader.js to trace the pre-load method queue, then compare its entries with the addIntegration implementation in packages/core/src/integration.ts. Confirm how queued calls are replayed after loading and verify that the chosen guardrail handles addIntegration without changing existing queued methods.

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