fix(math): Remove a recursion in TeX-like accent logic causing an error

[Omikhleia]

Placing an accent (say \bar) over a single macro-defined command (say a \Phi, or a \mathbf{x}) caused the engine to crash. The accent logic was incorrectly re-compiling the already compiled argument recursively. We were just lucky in other cases.

Closes #2291

Checks

(All of these were previously crashing)

$$`\Phi' \equiv \Phi - \overline{\Phi}`

$$`\bar{\mathbf{x}}, \bar{\mathbf{abc}}`

$$`\overline{\mathbf{x}}, \overline{\mathbf{abc}}`

Details

We were re-compiling an already compiled tree.
Most of the time this would do nothing:

• for most code paths, the "id" is stripped from the compiled tree in the final return, so most of the logic is just skipped if invoked on an already compiled node.
• but commands keep the "id" on their evaluated arguments / result, due to an early return in a long sequence of if-elseif... (I have not changed that, though, it's beyond my understanding.)

TL;DR. The key solution anyway is not to perform that useless re-compilation anyway.

Additionally, we were compiling accents from a constructed atom, but we actually know what we eventually want in MathML in that case, so it can be avoided (skipping another recursive evaluation).

My bad, when I implemented accents in Nov. 2024 🙄 (but I don't apologize, it was quite a struggle to get there).