docs(method): rewrite the Method page as prose and add the noisy-neighbours section

The Method tab read like internal engineering notes. Rewritten to read like a
piece someone would actually finish and trust: the rig, the one-config
promise, the 6x6 probe-then-sweep matrix, memory, and the honesty rules, all
verified against matrix/run.sh, lib/harness.sh, lib/box_qualify.sh and
run-on-ec2.sh so every claim matches what the code actually does.

Also adds the section the owner asked for: noisy neighbours. Explains that
the same gateway, same config, same instance type can still disagree between
runs because of shared-host contention, then explains the box-qualification
gate in lib/box_qualify.sh that catches it before a multi-hour run starts,
why its drift band is one-sided, and that the 4% band is provisional pending
recalibration on a frozen codebase.

Author: MattJackson