Proposal: Explicit FIX session state model for clearer protocol enforcement

[shan305]

While working with fixio, I noticed that FIX session state (before logon,
during logon, and after logon) is handled implicitly and spread across
different handlers.

Right now, the code mostly uses the presence of a FixSession object
(FixSession != null) to decide whether a session is “established”, even
though connection setup and logon exchange are still in progress.

I’d like to propose making the FIX session state explicit (for example,
using a small internal enum and clear state transitions), without
changing current behavior or adding new features.

The goal would be to:

• make the session lifecycle easier to understand
• avoid mixing connection, logon, and established states
• improve debugging and protocol correctness
• keep the current performance characteristics

This would be an internal change only and opt-in at first.

Before starting any implementation, I wanted to ask if this direction
fits the project’s goals.