feat: cheap KDoc lint in check; move dokka link-resolution off the critical path

[rasros]

What changed

• New checkKdoc verification task (lexical scan, like checkNativeSafeTestNames), wired into check. It flags:
  • unknown KDoc block tags — lowercase @tags not in the valid set (@param @return @throws …). PascalCase annotations (@Serializable, @Test) are ignored.
  • qualified [links] to a non-existent package — e.g. a typo'd [com.eignex.klause.nope.Thing]. Scoped to the project's own package root (longest common prefix), so sibling deps (com.eignex.skema) and stdlib ([kotlin.*]) are left alone. Handles top-level-function links; skips fenced code and markdown URLs.
• check no longer depends on lintDocs; it depends on the detekt tasks + checkNativeSafeTestNames + checkKdoc. Dokka's full link resolution moves to the dedicated lintDocs job (consumers already run it in parallel).

Why

lintDocs runs a full dokka generation (the slowest thing in CI) purely to validate KDoc. Detekt already enforces 'public symbols documented'; checkKdoc now covers tag validity + package-existence cheaply in check, so common KDoc mistakes are still caught on a local build. That removes the reason check had to drag dokka onto the critical path — dokka stays as the thorough resolution backstop in its own job.

Testing

./gradlew compileKotlin passes. The scan was validated against klause's full source: 0 false positives, and it correctly catches injected errors (bad @returns tag; […nope.Thing] and […propagatio.someFun] bad packages).