Flash Next CUDA admission refuses a shard in a folder inside the model folder (the family half of #1187) #1220

What happens. On main @ 8e49cf8 (and in the 1.1.2 release), the native CUDA server still refuses
nvidia/Qwen3.8-Flash-Next-FP4-Local: the publisher maps every tensor to
tensors/model-000NN-of-00021.safetensors. #1179 taught core/checkpoint_index.zig to accept a shard in a folder
inside the model folder, but Flash Next validates the weight map a second time before any header is read:

zig/src/families/flashnext/index.zig, admitWith:

if (file != .string or file.string.len == 0 or file.string[0] == '.' or
    std.mem.indexOfAny(u8, file.string, "/\\:\x00") != null or
    !std.mem.endsWith(u8, file.string, ".safetensors")) return error.UnsafeFlashShard;

so serve stops with BadIndex/UnsafeFlashShard before CUDA init unless the symlink-folder workaround from #1187
is used.

Why it is separate from #1187. #1179 is the core half and is already on main; this is the family half, on a
different code path, with its own regression surface.

Fix in the linked PR. A local shardName/plainPart in index.zig implementing the same rule as core's
shardName (plain names joined by /, no hidden or reserved part, must be .safetensors), so
tensors/model-…safetensors is admitted while ../, absolute paths, drive names, hidden parts and device stems stay
refused. Tests: the 30-shard fixture admitted flat, under tensors/, and refused under ../; a checkpoint whose
index names shards in a folder opens and reads them; pull lands the nested shard in its snapshot folder as a blob
link; info's size counts it.

Also found while doing this (all on unmodified main)

• zig build test-pull-security does not compile: hub.zig imports native_engines, which that step does not wire.
  A new test-pull-host step runs pull's tests with the engine registry and is part of test-cuda-host.
• zig/src/families/flashnext/host_test.zig is orphaned: no build step or module imports it, and it cannot compile
  standalone (a relative import outside the module path). Its admission tests never ran; equivalent coverage now
  lives in index.zig.