Add structured b2o persistence for logical objects and improve lazy/proxy semantics

[FrancescAlted]

Main Improvements:

• Added the new b2o metalayer as the primary source of truth for deserializing persisted CFrame-backed objects beyond plain SChunk / NDArray.
  This is now the structured persistence mechanism for objects such as C2Array, LazyExpr, and LazyUDF.
• Implemented structured b2object round-trips for higher-level Blosc2 objects.
  Persisted carriers can now reopen as their logical object type instead of remaining opaque array containers.
• Restored strict Proxy read-only semantics.
  blosc2.open(..., mode="r") on persisted proxies no longer mutates the local cache implicitly.
• Fixed saved lazy expressions to reopen local operands in the default open mode instead of forcing mode="r".
  This preserves direct read-only proxy semantics while allowing reopened lazy recipes to work correctly with proxy operands.
• Added LazyArray.vlmeta support.
  LazyExpr and LazyUDF now support user metadata both in memory and after persistence/reopen.
• Persisted lazy metadata round-trips cleanly.
  User vlmeta is stored separately from internal reconstruction metadata.
• Improved DictStore behavior for lazy objects.
  Persisted LazyExpr leaves are now discovered and reopened correctly.
• Fixed storage of lazy b2object carriers inside DictStore / .b2z bundles.
  Lazy recipes can now be bundled and reopened read-only without regressions.
• Updated docs and tests accordingly.
  This includes new regression coverage for proxy reopen semantics, lazy metadata round-trips, bundled lazy recipes, and platform-neutral path
  expectations.