Add doctests for Eval (closes #2479 in part)

[MavenRain]

Partially closes #2479.

Eval.scala had zero existing doctests despite Eval being the lazy /
memoized FP-flagship data type in cats. This PR adds scaladoc examples to
seven Eval methods:

• Eval#map, Eval#flatMap — basic mapping and chaining.
• Eval#memoize — confirms Always.memoize does not re-evaluate on repeated
  .value.
• Eval.now — eager evaluation.
• Eval.later — lazy with memoization; counter increments once across
  multiple .value calls.
• Eval.always — lazy without memoization; counter increments per .value
  call.
• Eval.defer — computation deferred until .value is forced.

The Later, Always, memoize, and defer examples use the var counter = 0 plus side-effecting block pattern already established in Defer.scala:39
and FlatMap.scala:88, to demonstrate the eager vs lazy-memoized vs
lazy-not-memoized distinction at the doctest assertion level.

Local validation

• ++ 2.13; coreJVM/test 916 / 916 pass (16 newly generated EvalDoctest
  assertions included).
• ++ 3; coreJVM/compile coreJVM/test clean (doctest generation is 2.13-only
  per build.sbt:59).
• scalafmtCheckAll clean.
• scalafmtSbtCheck clean.
• coreJVM/mimaReportBinaryIssues no binary issues.

A note for future Eval doctest authors

sbt-doctest silently skips scala> lines that contain a brace-lambda with a
;-separated body (e.g. Eval.now(1).map { x => mutate; x + 2 }). I caught
this on the first pass when map.example was missing from the test report
despite being a valid scaladoc block. The workaround is to either avoid ;
(use parens-lambda + a helper def) or to keep the example brace-lambda-free.