Testing — Unit tests that run your routines on an emulated CPU at build time. The shape of a !test block, what init_reg and assert_reg actually are, cycle budgets, and how to read a failure.
The Language Server — Errors as you type, go-to-definition, hover showing a label's value or an instruction's T-states, completion over your own names, project-wide rename, call hierarchy and quick fixes. How to point an editor at it, which file answered for the one you have open, and the one case where it can still disagree with a real build.
Rhai Scripting — Computing things at build time instead of typing them. The three forms a script takes, everything ctx offers, why an expression call cannot change state, the sandbox and its limits, and what a script fault looks like.
Using It as a Library — For Rust developers rather than Z80 programmers: assembling in-process and getting bytes and structured diagnostics back as values. Where the diagnostics actually live, why warnings is not only warnings, and what the crate promises about staying still.