Require tmux 3.3a for core operations #777

Summary

Adopt tmux 3.3a as the minimum for ordinary core operations and carry the same baseline through this project's callers, examples, documentation and CI. Keep newer optional-engine and feature requirements in a separate change.

Motivation

A version constant alone does not establish a support boundary. The connected daemon, executable used to start an absent server, raw dispatch and product entry points need consistent admission and refusal behavior.

Proposal

• Reject ordinary reads and writes below tmux 3.3a before the requested operation has an effect; preserve patch-letter ordering and generation checks.
• Update runtime callers and the required baseline matrix, and remove only adaptations made obsolete by the new floor.
• Retain compatibility code still needed for supported releases above the floor.
• Prove the negative gate with a deliberate break, then demonstrate supported baseline and newer-daemon behavior.
• Keep changelog changes in their own commit and review the final implementation independently.

References

• README.md.
• AGENTS.md.