server/item: move the item NBT codec out of nbtconv

[HashimTheArab]

Crossbow reaches the item stack NBT codec through go:linkname rather than an import, because nbtconv imports item and the import cannot go back the other way.

A linkname pull creates no dependency edge. The compiler never looks for nbtconv, and the linker only resolves the symbols if something else in the binary happens to import it. Anything importing server — a real server — gets it via server/entity and server/block, which is presumably why this has gone unnoticed. A binary that imports server/item on its own does not:

package main

import (
	"github.com/df-mc/dragonfly/server/item"
	"github.com/df-mc/dragonfly/server/world"
)

func main() {
	var nbt world.NBTer = item.Crossbow{}
	_ = nbt.EncodeNBT()
}

github.com/df-mc/dragonfly/server/item.Crossbow.EncodeNBT: relocation target
github.com/df-mc/dragonfly/server/internal/nbtconv.WriteItem not defined

It bites tools and test binaries that pull in server/item without reaching entities or blocks. I hit it writing an item-persistence helper package.

The change

The stack codec moves into item, where the type it serialises lives and where Crossbow can call it directly. That is the only direction the dependency can point: anything encoding a Stack must import item, so a package below item cannot do it, and item cannot import upwards.

All existing codec callers now use item.WriteNBT, item.ReadNBT, and item.MapNBT directly. The item-specific forwarding wrappers have been removed from nbtconv; its inventory helpers remain because they also encode and decode inventory slots. Both linknames and the now-unnecessary blank unsafe import are gone.

Net: 16 files changed, +364 / -320, almost all of it code moving between packages and direct call-site updates.

Two things worth a reviewer's attention

Duplicated map readers. The codec needs six trivial readers (Bool, Uint8, String, Int16, Int32, Slice) plus a block decoder. They are duplicated privately in item as nbtBool, nbtUint8 and so on rather than imported, because nbtconv depends on item. Splitting the generic half of nbtconv into a leaf package both could import would avoid that, at the cost of touching every call site — happy to do it that way instead if you prefer.

item picks up world/chunk, for chunk.CurrentBlockVersion in writeBlock. chunk does not import item, so there is no cycle, but it does widen item's footprint for one constant. Moving that constant somewhere more neutral would avoid it.

Testing

• go build ./...
• go vet ./...
• go test ./...
• The reproducer above fails to link on master and runs on this branch.
• mapValue keeps its name and field names. gob identifies the type by name, so worlds written before this still decode — the on-disk format is unchanged.