fix(nb1): use coherent btop CPU denominator

[ryanbreen]

Summary

• NB-1b verdict: PURE ARTIFACT, not a real two-core burn.
• Parallels /proc/stat idle-window deltas show CPU0 busy 28/2555 ticks (1.1%) and CPU2 busy 46/3470 ticks (1.3%); ARM64 idle paths are wfi-based.
• Fix btop process CPU accounting to read process ticks first, then use one coherent /proc/stat cpu_capacity_ticks/cpu_online snapshot for all process deltas.
• No timer, syscall, or interrupt hot-path files changed.

Evidence

• turn75-artifacts/nb1-groundtruth-and-fix-summary.txt
• turn75-artifacts/parallels-nb1-groundtruth.serial.log
• turn75-artifacts/parallels-nb1-after-fresh.serial.log
• turn75-artifacts/parallels-nb1-after-fresh.screenshot.png
• turn75-artifacts/final-userspace-aarch64-build-v2.log
• turn75-artifacts/final-kernel-release-build-v2.log

Validation

• Fresh Parallels ARM64 run rebuilt userspace/ext2; btop.elf installed at 294624 bytes.
• Fresh after run: max per-core 5%; max process 28.7% (bwm actively compositing), btop max 16.9%, heartbeat max 2.7%; no >=100.0% process CPU samples.
• Fault scan clean: no SOFT LOCKUP, CPU0 REGRESSION, DATA_ABORT, FATAL, panic, warning, or error matches.
• Final aarch64 userspace build warning/error grep empty.
• Final release kernel build warning/error grep empty.