Build robustness: surface build output, ERR-trap friendly failures, `-j` resource guard

[VijitSingh97]

Problem

cmake … &> /dev/null and make -j$CORES &> /dev/null (rigforge.sh:246-249) swallow all build output, so a failed compile aborts under set -e with zero diagnostics. There's also no ERR trap (pithead has on_err naming the step), and make -j$(nproc) can OOM the heavy XMRig build on low-RAM boxes.

Proposed

• Tee build output to a logfile (and/or don't suppress it); on failure point the user at it.
• Add a trap … ERR that names the failed step (mirror pithead's on_err).
• Cap build parallelism on low-RAM hosts (reserve headroom).

Acceptance

• Build failures show actionable output / a logfile path.
• An ERR trap reports which step failed.
• -j is bounded on low-memory machines.