docs(rules): QML int-width, binding-freshness, boundary-value gotchas

[jakewan]

Overview

The transaction-recording-form work surfaced three durable lessons worth encoding in the repo's own rule set so the next agent doesn't re-discover them rather than relying on one session's memory:

• A QML int is 32-bit, so a cents value — carried as int64 end to end through the daemon and proto — silently truncates above ~$214k. qt-app.md now documents typing any cents-bearing or int64-crossing property as var, not int.
• A derived property read inside the onXChanged handler of the property it depends on can return a stale, pre-change value. qt-app.md now records reading the changed source property directly in the handler.
• The overflow slipped every reading pass because example-based specs only probe representative magnitudes. testing.md gains a boundary-value convention — the orthogonal modality that catches that class — framed around type/representation/seam crossings so it complements the existing input-validation guidance rather than duplicating it.

The two qt-app.md sections are path-conditioned to app/**, so they cost nothing off-app; the testing.md section is cross-cutting (core/daemon/app seams) and always-loaded.

Issue references

Related to #38.