docs: the renderWithMantine rule no longer rests on timer safety

Review caught a second-order effect of the fix: making the leaked-timer net
global invalidated the rationale of a neighbouring rule.

"Do not hand-roll a bare MantineProvider — that reintroduces the leak class"
stopped being true the moment the net moved into setup.ts, which covers every
unit test regardless of how it renders. The rule itself is still right, so keep
it and replace the justification: a hand-rolled provider skips the project theme
and the helper's options and drifts from every other test. Both copies say
explicitly that the old reason no longer holds, so nobody re-derives it.

Mirrored into .github/copilot-instructions.md in the same PR, as AGENTS.md
requires for review-relevant guidance — it carried the identical stale claim.

That makes four places this one belief had propagated to. A fix can create stale
docs by making a previously-true statement false, which is easy to miss when
only the changed lines get read.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HDdo1rNRVQnVRRdsqSrvG3
Signed-off-by: cliffhall <cliff@futurescale.com>

.github/copilot-instructions.md

@@ -92,7 +92,7 @@ Both exist and do different jobs. Theme files (`src/theme/<Component>.ts`) custo
 
 ### Rendering components in tests
 
-- **Always render through `renderWithMantine`** from `src/test/renderWithMantine.tsx`. Never hand-roll a bare `MantineProvider` — that reintroduces a real failure class where a `Transition`/`Modal` timer fires after happy-dom tears down `window` and fails the entire run.
+- **Always render through `renderWithMantine`** from `src/test/renderWithMantine.tsx`. A hand-rolled bare `MantineProvider` skips the project theme and the helper's options, and drifts from every other test. (It does *not* reintroduce the timer-leak class — an older version of this rule said so; the leaked-timer net in `setup.ts` is global and covers every unit test regardless of how it renders.)
 - For a forced color scheme, pass the option — `renderWithMantine(ui, { colorScheme: "dark" })` — rather than a hand-rolled `defaultColorScheme` provider.
 - Only when asserting _mid-flight_ transition state, use `renderWithMantineTransitions`, passing `settleMs` derived from the component's real animation duration. Do **not** combine it with `vi.useFakeTimers()`, and use the `unmount()` it returns if the test unmounts the tree itself.