core: adapt to datascript Int64 values (pin e439cba)

[tiensonqin]

Summary

Datascript upstream c39cad1 changed value's Int of int to Int64 of int64 (merged as e439cba in logseq/datascript-ocaml). Main's #main pin picked it up mid-run and broke CI; this PR adapts chat's ~105 Ds.Int call sites and repins datascript to e439cba.

Conversion convention: chat-side domain types stay int; wrap at the Ds.Int64 boundary with Int64.of_int / Int64.to_int. Test literals use the L suffix. Entity ids remain int (Ds.Ref / Ref_to are unchanged upstream).

Notable non-mechanical bits:

• storage_codec.ml: Value.Int/Int64/Big_int all encode to Ds.Int64; decode accepts Transit.Int, Int64, and Big_int; Ds.Instant round-trips through Tagged ("m", ...).
• sync_tx.ml: emits Transit.Int64 unconditionally (upstream sends Int64 on the wire).
• cache_model.ml int_decode: Ds.Int64 → Int64.to_int, keeping created_at : int etc.
• graph_bootstrap.ml: file/created-at and file/last-modified-at now write Ds.Int64 instead of Ds.Instant — upstream storage.ml demotes Instant to Int64 on read for attrs without :db.type/instant, so storing Instant there no longer round-trips.
• Or-patterns Ds.Ref eid | Ds.Int eid were split into separate branches since Ref stays int while Int64 is int64.

Supersedes #25 (this branch contains its pin commit, then repins to e439cba).

Local: dune build @shared/native/runtest — 717 tests, all green.

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Requested by: @tiensonqin

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