sdk: ship the videre conformance kit with a wire-drift test gate

[mfw78]

Rename the venue conformance kit to videre-test and harden it so a venue's cargo test fails whenever its wire shape drifts. The kit exists as nexum-venue-test but is mis-named and its codec golden passes vacuously on an empty vector.

Why

The conformance kit is the gate that holds every venue to portable codec vectors and header goldens, but it is mis-named for the split and its codec golden currently passes on an empty vector, so drift can slip through. Renaming and hardening it makes wire-shape drift a hard cargo-test failure. Part of milestone M3: Videre SDK, macros and DX. Blocked by: videre-sdk: rename nexum-venue-sdk + add Keeper::sweep assembler + VenueClient. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Rename nexum-venue-test to videre-test; keep CodecVectors, HeaderGoldens and MockTransport.
• Harden the goldens with a version discriminator, reject-unknown handling and a non-empty-vector assertion.
• Regenerate the goldens under the videre:* namespace and re-assert the tip oracle.
• Align mock-grant fidelity to the host to close the known divergence.

Done when

• nexum-venue-test is renamed to videre-test.
• The kit ships CodecVectors, HeaderGoldens and MockTransport.
• A venue's cargo test fails on any wire-shape drift.
• The goldens carry the version discriminator, reject-unknown and a non-empty-vector assertion.
• The goldens regenerate under the videre:* namespace and the tip oracle holds.

[mfw78]

Landed via #459 (squash-merged into dev/m1 as 19edc65). Closing manually since the merge target is not the default branch.