Improvement: add flag to limit build jobs

[daleshort]

Adds an optional flag to the install script to optionally limit the number of parallel builds with cargo. I had difficulty getting this to install on a M1 Macbook air (8GB memory) normally and was getting memory errors ("SIGBUS: access to undefined memory") but passing -j 2 had no issues.

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Note

Low Risk
Low risk: changes are confined to install.sh, adding simple argument parsing and passing -j to cargo install to reduce parallelism; main risk is minor usability/compatibility issues in the script's new CLI handling.

Overview
install.sh now accepts an optional -j N argument, validates it as a positive integer, and forwards it to cargo install to limit parallel build threads.

The script builds the cargo command dynamically and improves failure guidance by suggesting retrying with ./install.sh -j 1 when builds fail due to memory pressure.

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