#134 docs(browser): clarify DOM global scope

Summary

• clarify that page globals are only available in session.Runtime.evaluate(...)
• distinguish the page context from the surrounding browser_execute snippet

Validation

• git diff --check

Closes #131

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Summary by cubic

Clarified bcode-browser docs: page globals (document, window, location) are available only within session.Runtime.evaluate(...), not in the surrounding browser_execute snippet. This removes ambiguity between the page context and the snippet context.