refactor!: rename GenServer to Actor across codebase

[ElFantasma]

Summary

• Rename all GenServer-related types and methods to more accessible Actor terminology
• Align APIs between tasks and threads modules
• Update all examples and documentation

Breaking Changes

Type Renames:

Old	New
GenServer	Actor
GenServerHandle	ActorRef
GenServerError	ActorError
CallMsg	Request
CastMsg	Message
OutMsg	Reply
CallResponse	RequestResponse
CastResponse	MessageResponse

Method Renames:

Old	New
handle_call()	handle_request()
handle_cast()	handle_message()
call()	request()
call_with_timeout()	request_with_timeout()
cast()	send()

File Renames:

• gen_server.rs → actor.rs (both tasks and threads modules)

New Features (threads module)

• request_with_timeout() using crossbeam's recv_timeout
• teardown() callback for cleanup
• init() now returns InitResult<Self> for consistency with tasks module
• send_message_on() sync equivalent of the tasks version

Documentation

• Added "Erlang to Spawned Terminology" section in README with mapping table
• Updated all code examples in READMEs

Test plan

• cargo build --workspace passes
• cargo test --workspace passes (26 tests)
• cargo clippy --workspace passes with no warnings

Closes #115
Closes #117
Closes #118
Closes #119
Closes #121
Closes #135

[Oppen]

This would be easier to review if the process had been:

1. git mv genserver.rs actor.rs
2. Modify actor.rs
3. git add actor.rs