docs: document build parallel limit - #2132

Greptile Summary

This PR adds a short troubleshooting tip to kt-kernel/README.md, placed immediately after the CPUINFER_PARALLEL environment-variable table. It explains how to retry a failed cmake --build step with a lower parallelism value on memory-constrained machines.

• Adds a prose explanation + bash code block showing CPUINFER_PARALLEL=6 ./install.sh build as a concrete retry example.
• Adds a follow-up sentence clarifying the trade-off between peak memory usage and build time.

Confidence Score: 5/5

Documentation-only change; no code paths are modified and no existing behaviour is altered.

The change adds nine lines of Markdown explaining an already-documented environment variable. The example command is correct, the prose is accurate, and the new content is placed logically next to the relevant table row.

Files Needing Attention: No files require special attention.

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Run ./install.sh build] --> B{cmake --build succeeds?}
    B -- Yes --> C[Build complete]
    B -- No / OOM error --> D[Set CPUINFER_PARALLEL=N where N < auto-detected]
    D --> A

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