feat: Add conditional marker exposure based on cross-origin isolation (supersedes #85)

[monica-ch]

Supersedes #85 (picking up the conditional-marker-exposure work from @HeathcliffAC — thanks for the initial draft).

This PR retains the original commit from #85 and adds one follow-up commit that:

Bug fixes

• Fix mangled JSON in the Trace with markers example in markers.md — the closing } was glued onto the same line as "timestamp": 150, breaking the code block.
• Correct the MSEdgeExplainers explainer URL — was github.com/MSEdgeExplainers/ConditionalMarkersExposure, corrected to github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/ConditionalMarkersExposure/explainer.md.
• Restore the newline between </p> and <section data-dfn-for="ProfilerSample"> in index.html that was collapsed by the previous edit.

Normative gap
The original PR replaced the [CrossOriginIsolated] IDL extended attribute with prose ("availability depends on the context''s cross-origin isolation status"), but never specified how the marker field is populated. That left two conforming UAs free to ship different subsets.

This adds a normative filter a marker algorithm in the Processing Model section, invoked from take a sample when setting ProfilerSample.marker:

To filter a marker given a ProfilerMarker marker and an environment settings object settings:
  1. If settings'' cross-origin isolated capability is true, return marker.
  2. If marker is "style" or "layout", return marker.
  3. Return null.

The <dfn>marker</dfn> prose is tightened to point at the algorithm rather than restating the exposure rule informally.

Opt-in hook
Adds a non-normative note reserving room for UA-defined opt-in mechanisms (e.g. origin trials, document policies) that may widen the set returned by filter a marker for a specific settings object, while still forbidding cross-origin marker exposure.

Implementation status

Chromium implementation of the algorithm as specified here is in flight https://chromium-review.googlesource.com/c/chromium/src/+/6012522. The filtering behavior in ProfilerTraceBuilder::GetMarker() — ProfileMarkerToPublicMarker — matches this algorithm exactly for the non-opt-in path.

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Preview | Diff

[monica-ch]

@acomminos @camillelamy @ArthurSonzogni Can I get your review on this change?

[acomminos]

Overall looks good to me, but I'd like @camillelamy / @ArthurSonzogni to provide some feedback on the security design choices here.