fix: preserve known cardinality when restoring stored sets

[RCmerci]

Restoring a stored set currently discards its known cardinality. A later count folds the entire tree, even when the caller has already persisted the exact count for that root snapshot.

Add a backwards-compatible ?count:int argument to restore and seed the existing immutable count cache. Existing callers that omit it retain lazy traversal-based counting. Reject negative counts without reading storage. The documented contract requires the caller to supply the exact cardinality of the addressed snapshot; the API does not scan nodes to verify a hint.

The existing successful add/remove paths already update a known count; duplicate inserts and absent removals preserve it. Tests cover those paths, empty sets, invalid counts, persistence of the original set, and the old no-count API. Melange and js_of_ocaml smoke tests exercise zero-read repeated counts too.

Validation on macOS arm64, OCaml 5.5.0, Dune 3.24.2, Node 22.21.1:

• A test written before the change failed because counting a cold restored 4096-element tree read 1365 stored nodes; the same test now passes with zero reads for repeated counts.
• Complete dune runtest --force --display=verbose: native, Melange/Node, js_of_ocaml/Node, Node GC and Chrome headless memory actions all passed.
• Strict upstream test-name coverage: 15/15; browser-memory CI wiring check passed. Name coverage is not behavioral equivalence.

A companion DataScript change passes the counts from its stored root before replaying the tail and pins this exact PSS commit. In the combined before/after probe, counts remain 4096/4096/2048 while both the first and second count change from 333 index-node visits / 10240 leaf values to zero visits. DataScript snapshot depth scanning is a separate cost and remains unchanged.

Local experiments use synthetic memory storage, not a personal graph. No heap or wall-time speedup is claimed.

Companion DataScript PR: logseq/datascript-ocaml#23 . It pins this PR head exactly; suggested merge order is PSS then DataScript.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-01T11:44:02.446354Z	9c99ee9	Draft marked ready

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