Wrap buffer-load and custom-loader Context constructors

[uqio]

Tracked under #6.

Scope

Context::new(path, params) is the only constructor today. Add support for loading a model from an in-memory buffer or via a custom whisper_model_loader.

Symbols

• whisper_init_from_buffer_with_params — load from &[u8].
• whisper_init_with_params — load via a user-supplied whisper_model_loader callback struct.
• whisper_model_loader (the callback struct itself: read / eof / close).

Why deferred

Both add lifetime / ownership complexity:

• Buffer load: the bytes need to outlive the load call but don't need to outlive the Context (whisper.cpp copies into its arena). API choice: &[u8] (borrowed for the call) vs Vec<u8> (owned, dropped after). Not hard, just hasn't been needed.
• Custom loader: the whisper_model_loader struct holds three function pointers + opaque context. Wrapping it ergonomically means a WhisperLoader trait + a C trampoline per method (similar to the abort-callback pattern). Larger surface, narrower use case (rare to need a custom loader vs buffer or path).

No caller has asked for either.

Acceptance

Buffer load is the smaller deliverable:

• Context::from_buffer(bytes: &[u8], params: ContextParams) -> WhisperResult<Self>.
• Same RAII / leak guarantees as Context::new.
• Audit-matrix row.
• Tests on a small in-memory model fixture.

Custom loader is a separate, larger deliverable:

• WhisperLoader trait (fn read(&mut self, buf: &mut [u8]) -> io::Result<usize>; fn eof(&self) -> bool; fn close(&mut self);).
• C trampolines for each.
• Context::from_loader(impl WhisperLoader, params) -> WhisperResult<Self>.

Probably one PR for buffer load, a second for custom loader.

If you have a use case for either (sandboxed model loading, network-streamed model bytes, model-in-an-archive), drop a comment.