Live Y::Map handles: read and edit shared maps from Ruby

[jpcamara]

Exposes the first yrs shared type for building and editing state from Ruby, not just opaque CRDT sync. Doc#get_map(name) returns a live Y::Map handle: reads reflect the current document and writes mutate the CRDT, so changes sync to every peer.

map = doc.get_map("state")
map["title"] = "Dashboard"
map["tags"]  = %w[a b c]
map["user"]  = { "name" => "Ada", "role" => "eng" }   # nested Y.Map

map["title"]            # => "Dashboard"
map.to_h                # => { "title" => "Dashboard", ... }

user = map.get_map("user")   # live handle to the nested map
user["name"] = "Grace"       # doc now has state.user.name == "Grace"

map.delete("title")     # => "Dashboard"

Full surface: []/get, []=/set, get_map, delete, clear, keys, size/length, key?/has_key?, to_h, each. Values round-trip primitives, arrays, and nested hashes (a nested hash becomes a real nested Y.Map). Symbol keys stringify. This complements the read-only read_text/read_xml/read_map snapshots.

Thread safety mirrors Y::Doc:

• Every operation opens its own transaction inside nogvl (GVL released) and holds no lock across the GVL boundary.
• Ruby values are read and built only with the GVL held; the closure works purely on Send data via an InValue intermediate, so no Ruby calls happen off-GVL.
• A handle is addressed by root name plus a path of keys and re-resolves on every operation, so it never caches a raw yrs branch pointer that could dangle when the tree is mutated. A nested handle keeps working as sibling keys change.
• Y::Map is asserted Send + Sync at compile time alongside Doc.

Tests: test/map_test.rb (29 tests) covers round-trips, assignment-returns-value, nested live maps, deep nesting, get_map on non-map/missing, live-handle-survives-reassignment, sync propagation to a peer, and an 8-thread concurrent read/write stress test. Rust unit tests cover the pure Send-side helpers (resolve, to_in, invalue_to_any). Full suite green: 201 Ruby runs, 38 Rust tests, clippy/rustfmt/rubocop clean. No version bump; the entry sits in the Unreleased section.

[jpcamara]

Rebased onto main (now includes the 0.2.3 delete-fix and 0.3.0 gap-free-sync releases) and bumped to 0.4.0 — 0.3.0 was taken by gap-free-sync. CHANGELOG slots [0.4.0] above [0.3.0]. Both feature sets coexist: full local suite green (121 Ruby, 28 Rust, clippy + rustfmt + rubocop clean).

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