Design: /system's memory chart pair uses the base grid utility, and sys-pair leaves §5d #3953

For: the design audit (#2678). One page part at a time onto §5's components (src/web/ui.ts helpers such as card, row, badge, and §5 classes), so docs/DESIGN.md §5d only shrinks.

src/web/pages/system.ts lays its RSS and JS-heap charts side by side in <div class="sys-pair">, with two rules in src/web/style/content.ts (a grid, and .sys-pair .chart-box { max-width: none; }).

Done:

• [ ] The pair is laid out with an existing base layout utility (.stack, .bar, or a §5 grid if one exists). Name the one you chose in the pull. Both charts still sit side by side from 680 px and stack below it, each at full column width.
• [ ] The .sys-pair rules are gone, and sys-pair leaves the style/content.ts bullet of §5d.
• [ ] Nothing a person reads or taps is lost. A test asserts the old class name is absent from the page's HTML, the page's existing tests pass, bun scripts/design-lint.ts passes (and --update gives a lower or equal baseline), and bun x tsc --noEmit passes before the push. Screenshots are not required; the sandbox has no browser.

Files: src/web/pages/system.ts, src/web/style/content.ts, docs/DESIGN.md, the test. Risk: #3866 edits the btop panels on the same page: keep to this pair. Leave .chart-box itself as it is (#3694 owns the chart).

Parked by the crew: the check stayed red through 2 automatic tries (pull #4029 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.

Split into #4146 (the markup) and #4147 (the stylesheet and docs), each one small pull that lands green on its own.