fix(#991): optimize atom-in-atom XSL with xsl:key

[ImilB]

Closes #991.

Problem

The atom-in-atom XSL transformation exceeds 100ms threshold on large XMIR files (~120ms).

Root Cause

The selector //o[eo:atom(.) and o[eo:atom(.)]] calls eo:atom(.) on every <o> element in the document and then again on each of their children. eo:atom(.) = exists($o/o[@name=$eo:lambda]) performs a child element lookup each time. The effective cost is O(n × degree), which on large XMIR files with many objects and children exceeds the threshold.

Solution

Added xsl:key for fast lookup of atoms and replaced the expensive function calls.

[volodya-lombrozo]

@ImilB Thank you for the contribution! But these changes require some sort of a test. It would be nice to have some proof that these changes are actually improving performance

[ImilB]

@volodya-lombrozo Thank you for the review!

I've added performance test results:

Performance Improvement

Metric	Before	After	Improvement
Time	120ms	45ms	75ms (62.5%)
Status	❌ Over threshold	✅ Under threshold	-

How to reproduce

Run the lints against eo-runtime:

mvn clean install -Pqulice --errors -Dstyle.color=never
Before
[WARNING] XSL transformation 'atom-in-atom' took 120ms, whereas threshold is 100ms
After
[INFO] XSL transformation 'atom-in-atom' took 45ms, whereas threshold is 100ms

The optimization works as expected.
<img width="2137" height="1192" alt="изображение" src="https://github.com/user-attachments/assets/19617f0d-359d-4bed-a07b-42258f5d51d7" />

[ImilB]

[ImilB]

@yegor256 solved #991