[Rust] IndexParams/SearchParams setters silently accept invalid values — add validated builder

Environment

• cuVS: 26.2 (crates.io)
• CUDA: 12.4
• GPU: T4 (sm_75)
• Rust: 1.82 (MSRV)

Problem

IndexParams setter methods return Self unconditionally. Invalid parameter
values are accepted at configuration time and only surface as an opaque CUDA
assertion inside Index::build() — after GPU memory has already been allocated
and graph construction has started.

// This compiles. It runs. It says nothing.
// ~1.8 seconds later, during Index::build():
let params = IndexParams::new()?
    .set_graph_degree(0)               // graph_degree=0 is invalid for CAGRA
    .set_intermediate_graph_degree(0)  // must be >= graph_degree
    .set_nn_descent_niter(0);          // must be > 0 for convergence

// Error surfaces here:
// cuvsError { code: CUVS_ERROR, text: "raft::exception: assertion failed" }
// No field name. No valid range. No hint.
let _index = Index::build(&res, &params, &dataset)?;

The same problem affects SearchParams (set_hashmap_max_fill_rate(1.5),
set_team_size(7)) and IndexParams for ivf_pq, ivf_flat, and vamana.

When CAGRA parameters are loaded from a config file at runtime, a
misconfiguration blocks the GPU thread for the full build duration before
producing an error that names neither the offending field nor its valid range.

Root Cause

The C API (cuvsCagraBuild) validates parameters on entry and returns
CUVS_ERROR on failure. The Rust bindings expose setters that write directly
to the FFI struct without pre-validation, so constraints documented in the C
header are invisible to callers until Index::build() fires.

Proposed Fix

Add IndexParams::builder() — a parallel, additive entry point that validates
all parameters in a single build() -> Result<IndexParams> call before any
FFI allocation occurs. Existing IndexParams::new()?.set_graph_degree(...) is
unchanged.

pub struct IndexParamsBuilder { /* private */ }

impl IndexParamsBuilder {
    pub fn graph_degree(mut self, v: usize) -> Self;
    pub fn intermediate_graph_degree(mut self, v: usize) -> Self;
    pub fn nn_descent_niter(mut self, v: usize) -> Self;
    // ... remaining fields

    /// Validate parameters in pure Rust (no GPU needed).
    pub fn validate(&self) -> Result<()>;

    /// Validate all parameters and allocate the FFI struct.
    ///
    /// Returns `Err` with a message naming the offending field and its valid
    /// range before any GPU work begins.
    pub fn build(self) -> Result<IndexParams>;
}

impl IndexParams {
    pub fn builder() -> IndexParamsBuilder;
}

Usage:

let params = IndexParams::builder()
    .graph_degree(32)
    .intermediate_graph_degree(64)
    .nn_descent_niter(20)
    .build()?;
// Err("graph_degree must be > 0; got 0") fires immediately,
// not 1.8s later inside Index::build()

Validation rules (derived from C header constraints):

• graph_degree > 0; multiple of 32 preferred for warp alignment
• intermediate_graph_degree >= graph_degree
• nn_descent_niter > 0
• itopk_size must be a power of 2
• hashmap_max_fill_rate ∈ (0.1, 0.9)
• team_size ∈ {0, 4, 8, 16, 32}

Backward Compatibility

The existing IndexParams::new() API and all setter methods are unchanged.
The builder is purely additive. No call sites break.

Scope

Rust bindings only. No C API changes required. The validation logic duplicates
constraints already enforced by the C layer — we are surfacing them earlier,
before the FFI call, with useful error messages.

A proof-of-concept implementation is available at:
https://github.com/zbennett10/cuvs/tree/feat/rust-validated-index-params-builder

Files

• rust/cuvs/src/error.rs — add impl From<String> for Error
• rust/cuvs/src/cagra/index_params.rs — IndexParamsBuilder
• rust/cuvs/src/cagra/search_params.rs — SearchParamsBuilder
• rust/cuvs/src/cagra/mod.rs — re-export builder types
• Apply same pattern to ivf_pq, ivf_flat, vamana for consistency

Start with rust/cuvs/src/cagra/index_params.rs, rust/cuvs/src/cagra/search_params.rs, rust/cuvs/src/cagra/mod.rs, and rust/cuvs/src/error.rs to understand the existing setters, error type, and exports. Compare the C-header constraints listed in the issue with the CAGRA, IVF-PQ, IVF-flat, and Vamana parameter APIs. Done means additive builders validate before FFI allocation, return field-specific errors, and preserve the existing setter API.