Classify missing subpaths from the importer, not the workspace root #52

Follow-up to #51 (raised by Codex review there).

The not-installed classifier (shared/missingDependency.ts, used by Rstest since #29 and by Rslint/fmt since #51) proves that a bare specifier's package is absent by walking up the physical node_modules from a fixed root: the project root (Rstest), the config directory (Rslint) or the workspace root (fmt).

That proof is wrong for one shape: a config imports plugin p, p has a private dependency dep installed only under p's own node_modules (isolated pnpm layout), and p requests a nonexistent subpath dep/missing. The walk-up from the root cannot see dep/package.json, so a real missing-subpath error is classified as "not installed": the stack goes disabled and polls instead of surfacing the actionable error.

Fix direction: resolve dep from the importer the loader message identifies (ESM: imported from <file>; CJS: the first Require stack entry) before declaring it absent, falling back to the current root walk when no importer is present in the message.

Pre-existing for Rstest; #51 extended the same limitation to Rslint and fmt rather than introducing it.