fix: compare extern references without an equals method by identity in the JS runtime

[fabiomadge]

Part of #6491, JavaScript variant — the sibling of #6497 (Go).

The runtime's universal equality _dafny.areEqual sends objects that are not Dafny-generated classes (no _tname marker) to a.equals(b). Extern class instances carry no _tname, so:

• an ordinary extern class with no equals method crashed any generic comparison — var s := {a, b} died with TypeError: a.equals is not a function;
• an extern with a structural equals silently subverted verified reference equalities, as on the other backends.

Fix. A 3-line arm in the dispatch: objects without an equals method compare by identity — the equality Dafny gives reference types. Defining an equals method remains the structural opt-in for extern value types, the same contract as Go's EqualsGeneric (#6497). Because JS collections are linear scans through this same dispatch, set/map/seq operations over method-less externs become sound too — no hashing discontinuity exists on this backend.

The behavioral delta is confined to programs that previously crashed. Probed inhabitants of the changed arm: extern classes without equals, plain object literals, host objects (Date), and typed arrays all previously threw and now compare by identity; every equals-bearing object (including all runtime value types) is dispatched exactly as before. Survey of in-repo JS externs (5 files): all define classes with no equals — each was one generic comparison away from the crash; none relies on structural equality.

One nuance for review: the check is per-object capability (typeof a.equals !== "function"), not per-type — an extern inheriting equals through its prototype chain counts as opted in. That is the only type boundary JS offers, and it matches the pre-existing structural dispatch.

The regression test uses a method-less extern class and exercises set construction and membership (previously the crash site).

Scope: the crash was reachable without externs

The new arm is not reached only by extern references. Two native reference kinds are plain JavaScript objects with no equals method — a multi-dimensional array (_dafny.newArray returns a bare {dims, elmts} literal, with no _tname either) and an iterator. Comparing either generically fails on master with TypeError: a.equals is not a function:

var a := new int[2, 2];
var b := new int[2, 2];
var s: set<array2<int>> := {a};   // crashes on master
print a in s, " ", b in s, "\n";  // this branch: true false

Verified against a master build: it crashes; this branch prints true false and verifies. git-issue-6491-js-native.dfy covers both kinds, with no extern code and no --input shim, and the news fragment has been widened accordingly — it previously described the fix as extern-only.

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