refactor(go bindings): thread safe contexts over whisper state

[ciricc]

Problem

• Context was params-centric and shared global state, leading to racy access and "concurrent map writes".

Solution

• Refactor Context to be state-backed. Each Context now owns its own whisper_state for safe concurrent processing.
• Introduce stateful and stateless Context flavors.
• Add a high-level Parameters wrapper with a unified interface.

Key changes

• Context isolation: per-instance whisper_state ownership.
• Parameters abstraction: new top-level Parameters type.
• Resource management: Close methods added and state validity checks after Close.
• Type safety: wrap whisperCtx and whisperState pointers with lifecycle management.
• Concurrency: fix sync.Map usage to be safe for parallel processing (since 59b3c47).
• Tooling and tests: new benchmarks, test helpers, and expanded test coverage.
• Build and module updates: Makefile and go.mod adjustments.

Compatibility

• Backwards compatible: existing methods continue to work, with deprecation warnings where applicable.
• Concurrent safe: removes "concurrent map writes" under parallel workloads.

Migration notes

• Prefer the new Parameters wrapper and the new stateful/stateless context constructors.
• Always call Close on Context and related resources when finished.