Harden v2 backward compatibility

[instafluff]

What changed

• adds a dedicated v1 compatibility boundary for the public ComfyJS bundle
• restores legacy chat delivery semantics for highlighted messages, channel-point messages, /me actions, and @mention !command commands
• freezes legacy onJoin, onPart, and onRaid payload shapes
• preserves legacy cheer callback behavior without also firing onChat
• adds regression tests for these compatibility contracts
• adds secret-free CI across Node 20, 22, and 24 with typecheck, tests, build, and package verification

Why

The existing v2 branch had strong parser/unit coverage, but it did not have an explicit compatibility boundary. Several later changes had drifted from v1 behavior even after parity fixes were documented. This PR makes backward compatibility executable rather than relying on a checklist.

Next in this PR

• expand the compatibility oracle across the rest of the v1 API and callback payloads
• verify package/browser/CommonJS import surfaces
• add modern EventSub events through additive APIs without mutating legacy callbacks
• add live/captured differential validation

Draft while compatibility coverage is being completed.