Add an opaque userData slot to VecSimDiskContext

[kei-nan]

VecSimDiskContext is a pass-through: the embedder fills it, vecsim forwards it to the disk index, and the storage layer behind storage is what interprets the contents.

Storage implementations that keep several indexes in one physical store need to know which of them a handle belongs to. The index name is already available, but it is variable-width, which makes it a poor fit for a key prefix.

This adds uint32_t userData for that purpose — forwarded unchanged and never interpreted here.

Two deliberate choices:

• A value, not a pointer. storage is a void * because it is genuinely a handle to a live object; this is a small identifier, so a pointer would import lifetime and ownership questions across the boundary for no benefit.
• 32 bits, sorted into the struct by width. Placed above rerank so fields run widest-first, which leaves VecSimDiskContext at 32 bytes — the same size as before this PR, so the slot costs nothing. The only value it carries today is a 16-bit field index; stopping at u32 keeps headroom without pinning the embedder's current width into a public struct, and narrowing to u16 would shrink nothing, since the trailing bool pads both widths out the same.

Reordering is safe: every caller builds this struct with designated initializers, and none sets userData yet. Checked with static asserts that sizeof(VecSimDiskContext) == 32, offsetof(userData) == 24, and that a C++20 designated initializer in the new order still compiles.

The meaning of the value is documented on the consumer side, where it is interpreted, rather than here.

Self-contained: no other change is required for this to be correct, and nothing in this repository reads the field.

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Note

Low Risk
Additive public struct field with no in-repo consumers or behavior changes; callers must zero-initialize new fields when constructing the struct.

Overview
Extends the public VecSimDiskContext struct with a uint32_t userData field so embedders can pass a fixed-width index identifier into the disk storage layer (e.g. when several logical indexes share one physical store), without relying on variable-length indexName for keying.

VecSim does not interpret userData; it is only documented as forwarded unchanged with the rest of the disk context. rerank is unchanged aside from comment formatting.

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