feat(otel): propose browser behavioral analytics semantic conventions

## Summary

Propose OTel semantic conventions for browser behavioral analytics to the OpenTelemetry specification.

## Proposed Conventions

| Attribute | Type | Description |
|---|---|---|
| `browser.dom.target` | string | CSS selector of interaction target |
| `browser.dom.path` | string[] | DOM path from target to root |
| `browser.event.type` | string | DOM event type (click, keydown, etc.) |
| `browser.session.id` | string | Behavioral session identifier |
| `browser.viewport.width` | int | Viewport width in pixels |
| `browser.viewport.height` | int | Viewport height in pixels |
| `browser.pointer.x` | double | Pointer X coordinate |
| `browser.pointer.y` | double | Pointer Y coordinate |

## Process

1. Validate mapping via Flagon's OTLP exporter (#140)
2. Draft OTEP (OpenTelemetry Enhancement Proposal)
3. Submit to opentelemetry-specification repo
4. Iterate based on SIG feedback

## Depends On

- #140 (OTLP exporter — proves mapping works in practice)
- Community validation via demo (#141)

Start by reviewing #140 for the OTLP exporter mapping and #141 for the demo's community validation. Use those results to draft the OTEP and submit it to the opentelemetry-specification repository; done means the proposal has been submitted and iterated based on SIG feedback.

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