[Feature request] Inline assertions whose success/failure result can be used in expressions

Spinoff from the comments of #201. I've encountered a use case for this twice recently:

- [In pokered](https://github.com/pret/pokered/pull/557#discussion_r2656596307), we twice want to do `assert COND, "message"` but then use `COND` in some further logic. Having to define a temp variable would be more trouble than it's worth vs just repeating the `COND` expression.
- In pokecrystal, we repeat this pattern for getting an unlabeled SRAM location from its labeled WRAM counterpart:
```asm
ld hl, sPlayerData + (wEventFlags - wPlayerData)
ld hl, sPlayerData + (wHallOfFameCount - wPlayerData)
ld hl, sPlayerData + (wPlayerID - wPlayerData)
; etc
```
What I'd rather do:
```asm
def Saved(label) = sPlayerData + (label - wPlayerData) ; user-defined function
ld hl, Saved(wEventFlags)
ld hl, Saved(wHallOfFameCount)
ld hl, Saved(wPlayerID)
; etc
```
However, that would be unsafe to use without a precondition of `label >= wPlayerData && label < wPlayerDataEnd`. User-defined functions are single expressions, so we would need an expression `assert`:
```asm
def Saved(label) = \ ; user-defined function
assert(label >= wPlayerData && label < wPlayerDataEnd, "Cannot save label") * \
sPlayerData + (label - wPlayerData)
ld hl, Saved(wEventFlags)
ld hl, Saved(wHallOfFameCount)
ld hl, Saved(wPlayerID)
; etc
```

We could probably also handle three-arg `assert(warn/fail/fatal, cond, msg)`.

No implementation files, entry points, or tests are named in the issue. Start by reviewing the existing assert syntax and user-defined function handling, then define how one- and three-argument expression assertions should behave; done means the assertion result can be used in an expression with coverage for success and failure cases.

Written by the indexing model from the issue text.