perf: move every rarely-updated `Core.Context` field into the cold subobject

This PR cuts the reference count traffic paid by the recursion-depth and `withRef` steps that dominate elaboration, from ten increments per reconstruction to three. Measured against master it saves between 0.35% and 1.24% of elaboration instructions across match-, simp- and grind-heavy workloads.

`withReader` can never reuse the `Context` record, so every reconstruction pays one increment per pointer field. The original split classified fields by their number of update *sites*; this classifies them by measured update *frequency* instead. On a representative module `withIncRecDepth` runs about 1.0M times and `withRef` about 31k, while `withOptions` runs once, so `options`, `maxRecDepth`, `currNamespace`, `openDecls`, `initHeartbeats`, `maxHeartbeats` and `currMacroScope` join the cold group and only `currRecDepth` and `ref` stay hot. The `Context` constructor goes from `(0, 10, 2)` to `(0, 3, 2)`.

Rebuilding the wider cold record does make the rare paths that update those fields more expensive: `withCurrHeartbeats` goes from ten increments to fourteen. At roughly 4.9k such calls against 1.05M reconstructions, the trade is strongly positive.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>