Refactor: namespace all classes under Absmartly module

[Copilot]

All 55+ classes were defined at the top level (e.g., Client, Context, Unit), polluting the global Ruby namespace and causing collisions in consuming apps using autoloaders like Zeitwerk.

Changes

• Moved all lib files from lib/ into lib/absmartly/, following standard gem layout convention (lib/<gem_name>/)
• Wrapped all classes/modules in module Absmartly — lib/json/ → lib/absmartly/json/, lib/json_expr/ → lib/absmartly/json_expr/
• Preserved monkey-patches unwrapped: String#compare_to in string.rb and Array.wrap in expr_evaluator.rb stay outside module Absmartly
• Updated entry point lib/absmartly.rb require paths from "client" → "absmartly/client", etc.
• Updated json_expr/json_expr.rb operator requires from 'json_expr/operators/...' → 'absmartly/json_expr/operators/...'
• Updated all 30+ spec files to use Absmartly::ClassName references and updated require paths
• Version bump 1.3.0 → 2.0.0 (breaking API change)

Before / After

# Before — top-level, collides with consuming app constants
client = Client.create(config)
ctx = Context.create(...)

# After — properly namespaced
client = Absmartly::Client.create(config)
ctx = Absmartly::Context.create(...)

The Absmartly module-level convenience methods (Absmartly.configure_client, Absmartly.create_context, etc.) are unchanged.

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