Add structuredClone fallback for older browsers

[mozzius]

Summary

• fall back to JSON cloning when structuredClone is unavailable
• preserve deep-clone behavior for rich-text facets
• add a patch changeset for @bsky/sdk

Test plan

• In a browser without structuredClone, clone rich text with a facet and confirm mutating the clone does not affect the original.

[claude]

Claude finished @mozzius's task in 59s —— View job

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Review: Add structuredClone fallback for older browsers

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No blocking findings — this PR looks good to merge.

The change is correct and appropriately scoped. Notes on what I checked:

• JSON round-trip is equivalent here. Facet is app.bsky.richtext.facet.Main, a lexicon-generated plain-object type (index + features, all JSON-serializable). It has no Date, Map, Set, undefined-valued properties, or cyclic references, so JSON.parse(JSON.stringify(...)) produces the same deep clone structuredClone would. The mutation-isolation test confirms the deep-copy behavior.
• The undefined guard is necessary and correct. this.facets is optional (Facet[] | undefined). structuredClone(undefined) returns undefined, but the fallback's JSON.parse(JSON.stringify(undefined)) would throw (JSON.stringify(undefined) → the value undefined → JSON.parse on it is a SyntaxError). The early typeof value === 'undefined' return handles this, and the test's new RichText({ text: 'test' }).clone().facets assertion covers it.
• Feature detection is safe. typeof structuredClone === 'function' on a possibly-undeclared global doesn't throw, and the test stubs it to undefined to exercise the fallback path.
• Consistency: this was the only structuredClone usage in @bsky/sdk (both clone() and copyInto() now route through cloneDeep), so nothing is left on the old path.
• Changeset: patch for @bsky/sdk is the right bump — bug fix, no public API/type change.

One minor, non-blocking thought (take it or leave it): JSON.parse(JSON.stringify(...)) silently drops any future non-serializable facet field. That's a non-issue for today's lexicon-defined Facet, but if facets ever grow richer shapes, a structural clone helper would be more future-proof. Not worth changing now.
· samuel/fix-structured-clone-fallback