fix: restore declarations for incompatible selectors after rule merge

[PranavAchar01]

Summary

Fixes #1260.

When a style rule contains a mix of target-compatible and target-incompatible selectors, merge_style_rules merges the compatible portion into the preceding rule by draining style.declarations. This left the clones created for each incompatible selector with empty declarations; they were then silently discarded by the rule.is_empty() guard.

Reproduction:

/* Input — targets: Chrome 104 (:has() incompatible) */
.a:last-child { border-bottom: none; }
.a:last-child, .a:has(+ .b) { display: flex; }

Before fix:

.a:last-child { border-bottom: none; display: flex; }
/* .a:has(+ .b) { display: flex } was silently dropped */

After fix:

.a:last-child { border-bottom: none; display: flex; }
.a:has( + .b) { display: flex; }

Fix: snapshot style.declarations before the merge attempt and restore the snapshot into each incompatible-selector clone.

Test plan

• cargo test passes (119/119 lib tests)
• Added regression test using prefix_test with Chrome 104 targets that would have failed before this fix

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