drainStderr discarded all server stderr, so an LSP init failure (e.g. 'command not found', missing module) surfaced as an opaque 'initialization failed'. Retain a bounded tail (StderrCaptureMax=4096B, oldest dropped) under a mutex and attach it to the init-failure error context. Adds TestClientStderrCapture.
We advertise textDocument.definition.linkSupport=true, so gopls and rust-analyzer return the LocationLink shape (targetUri/targetRange). The old code only tried []Location then a single Location; unmarshaling LocationLink JSON into []Location silently succeeds with zero-value URIs/ranges, producing garbage locations (or a false 'No definition found'). parseDefinitionResult now decodes a unified shape capturing both field sets and normalizes to Location, and surfaces a real parse error instead of swallowing it. Adds TestParseDefinitionResult covering all shapes.
lukaszraczylo