Move error classification/absorption modules out of the components tree

## Problem

The absorption/classification modules introduced by #14440 are pure domain logic but live under `src/components/rightSidePanel/errors/`:

- `missingResourceAbsorption.ts` (matching policy)
- `errorSeverityClassification.ts` (aggregation)
- `useErrorClassification.ts` (reactive wrapper)

`executionErrorStore` now imports the matcher from the components tree for its retirement watcher — a store depending on a components path (precedent exists via `appModeStore`, but the direction is inverted). The panel folder name also under-sells the consumers: the run button, tab icon, and error overlay all classify through it.

## Suggested move

Relocate the three modules (and their tests/fixtures) to a domain layer — e.g. `src/core/errors/` or `src/platform/errors/` — leaving the presentation-only `useErrorGroups`/components in place. Mechanical, but deferred out of #14440 to keep that diff reviewable.

Start with src/components/rightSidePanel/errors/missingResourceAbsorption.ts, errorSeverityClassification.ts, and useErrorClassification.ts, then trace their tests and fixtures and the executionErrorStore import. Move the domain modules and related tests/fixtures to a suitable errors layer while leaving useErrorGroups and presentation components in place; update consumers so tests still pass.

Written by the indexing model from the issue text.