Clarify visible-page semantics

[Driedsandwich]

Summary

• define visible-page content as CSS-rendered or potentially visible
• document the hidden/inert, display, visibility, and opacity exclusions
• state that viewport presence, lack of occlusion, and current on-screen presence are not guaranteed
• keep viewport clipping, IntersectionObserver checks, and occlusion detection outside the runtime design

Closes #14

Scope

This PR addresses GPTPro re-audit finding F004 only. F003, runtime extraction, permissions, dependencies, workflows, releases, and packaging are unchanged.

Verification

• bundled Node test — 40/40 passed
• repository sanity static checks — passed
• git diff --check — passed
• docs truth search — rendered/potential visibility and viewport/occlusion limitations present
• security triage — docs-only two-file diff; no runtime, manifest, workflow, package, permission, or communication-path changes

Boundaries

• no IntersectionObserver, viewport clipping, or occlusion detection
• no F003 changes
• Draft and unmerged pending maintainer review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Qualify the descendant visibility guarantee

When a rendered body or comment contains a nested opacity: 0 or [inert] element, isVisiblyRendered checks only the extraction root and its ancestors before reading the root's complete innerText; opacity and inert do not remove that descendant text from innerText. The new wording therefore promises an exclusion the implementation does not enforce. Either limit the claim to body/comment extraction roots and their ancestors or filter hidden descendants before export.

AGENTS.md reference: AGENTS.md:L62-L73

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