feat: add drag-and-drop event support (lvt-on:drag*)

[adnaan]

Context

Identified during the patterns example proposal — see livetemplate/docs/proposals/patterns.md (Future Features section) and the Sortable / Drag-and-Drop entry.

The Sortable pattern (#32 in the patterns proposal) is skipped until drag-and-drop event support lands. Without native lvt-on:drag* bindings, implementing reorder requires custom JavaScript, which violates the "no custom JavaScript" principle for examples.

Requirements

Client:

• Add lvt-on:dragstart, lvt-on:dragover, lvt-on:drop event support (mirroring existing lvt-on:click etc.)
• Serialize drag source / target data in the outgoing action message — the controller needs to know which item moved, where from, where to

Core (livetemplate library):

• Reorder protocol for range items — extend existing range operations (["o", ["id1", "id2", …]] already exists for reorder; the new event surface needs to flow into it without bespoke patching)

Out of scope

• Touch drag (HTML5 drag-and-drop API only for v1)
• Custom drag previews / drag images
• Drop zones across iframes

References

• htmx Sortable example — the pattern this unblocks
• Patterns proposal Future Features section: livetemplate/docs/proposals/patterns.md
• Existing range reorder operation: livetemplate/internal/diff range ops (consumer side already supports ["o", …])

A complementary library issue can be filed if the protocol changes are non-trivial; the client side is the primary surface area.