Ground truth: numcrux transfer/donation (governance decision) #987

Sub-issue of #984.

skainet-ground-truth's actual Python content is already framework-agnostic — it
imports only torch and gradienttracer, zero SKaiNET/Kotlin coupling. The "SKaiNET"
branding is only in the repo name and README framing, not the code, matching
numcrux's own stated mission ("a public, neutral,
versioned reference-data commons... framework-agnostic and language-agnostic... will
start with the existing numerical test suite from SKaiNET").

skainet-test-groundtruth/build.gradle.kts's fixture-source path is now overridable
(-PgroundTruthSourceDir, defaults to the sibling-checkout convenience it always had),
and CONTRACT.md documents the GGUF tensor-naming/metadata conventions explicitly — so
SKaiNET's consumer no longer needs to change if/when the underlying fixtures move to a
numcrux-owned repo. That was the technical precondition; this issue is the remaining
non-technical one.

Open questions (for whoever owns this decision)

• Does skainet-ground-truth's content get donated/transferred to the numcrux org
  outright, or does numcrux start a fresh corpus and skainet-ground-truth stays as
  SKaiNET's own (possibly narrower, possibly diverging) fixture set?
• If transferred: does gradienttracer (the execution/serialization engine) move too,
  or stay under SKaiNET-developers as infrastructure numcrux depends on?
• License/attribution framing for a neutral commons vs. a SKaiNET-branded repo.
• Whether SKaiNET's CI (.github/workflows/ground-truth.yml) should point at the
  numcrux-hosted corpus once one exists, via -PgroundTruthSourceDir, or continue
  using skainet-ground-truth as a curated subset.

Not blocking anything today — filed so the decision has a home when it's ready to be
made, and so the "near future" framing from the original ask has a concrete place to
land.