perf(start): compact deferred hydration private state (#7980)

* perf(solid-start): shorten private prefetch state

* perf(start): use locals for hydration wait state

* perf(start): compact visible observer registry entries

* test(start): cover hydration private-state lifecycles

* docs: record solid hydration bundle result

* Delete RESULT-optimization-solid-hydration-private-state.md

* changeset

* ci: Version Packages (#7993) [Self-Healing CI Rerun]

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