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feat(immut/hashmap): add HashMap constructor, deprecate from_array

The immutable `HashMap` was a positional newtype `struct HashMap(Node?)`,
whose tuple constructor reserved the `HashMap` name, so a `HashMap::HashMap`
function could not be defined.

Give the field a name instead:

    struct HashMap[K, V] { data : Node[K, V]? }

This frees the `HashMap` identifier (named structs construct via `{ data: ..}`,
not `HashMap(..)`) while keeping the exact same `Node?` representation, so the
internal logic is unchanged apart from `self.0` -> `self.data` and wrapping
results in `{ data: .. }`. The public type stays abstract (the field is
private), so its `.mbti` is unchanged apart from the new constructor.

Add `HashMap::HashMap(ArrayView[(K, V)]) -> HashMap[K, V]` so a map can be
built with `@hashmap.HashMap([...])`, deprecate the old `from_array` (and its
`of` alias) in favour of it, and migrate all usages across the package. The
`Show`/`output` format string and its snapshot tests intentionally keep
emitting `@immut/hashmap.from_array([...])`.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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