Make native contrib scan dispatch generic (remove per-contrib arms from core Rust)

[schenksj]

Follow-up from review of #4952 (thread).

Background

On the JVM side, #4952 makes contrib scan wiring fully generic: CometScanContrib is a ServiceLoader-discovered SPI, core holds no compile-time reference to any contrib, and a default build discovers nothing.

The native side is nearly there. Core's dispatcher arm is already generic — it matches OpStruct::ContribScan and routes on type_url, and every Delta-specific concern (the type name, the decode, the planning) lives inside a #[cfg(feature = "contrib-delta")] module:

OpStruct::ContribScan(contrib) => {
    #[cfg(feature = "contrib-delta")]
    if let Some(result) = delta_scan::try_plan_contrib_scan(self, spark_plan, contrib) {
        return result;
    }
    Err(GeneralError(format!(
        "Received a contrib_scan operator (type_url: {}) but core was built without a \
         contrib that handles it. ...", contrib.type_url)))
}

What remains is that core still names each contrib: one #[cfg]-gated call per contrib, growing by a line as contribs are added. A registry of type_url -> handler that contribs populate would remove even that.

Two paths (from the review)

1. A true ServiceLoader-style system — dynamic discovery and loading of an extension at runtime. "There may be dragons along this path." Reference: https://nullderef.com/blog/plugin-dynload/
2. Statically linked, independently built crates — each contrib crate builds on its own and registers into core's dispatch table. Likely needs crate-level refactoring to avoid a core -> contrib -> core cycle (core currently owns the proto→arrow schema converter the contrib needs, which is why the thin Delta shim lives in core rather than in the contrib crate).

Priority

Low. Unlike the JVM side, this coupling is compile-time and feature-gated: a default build links zero contrib symbols, which dev/verify-contrib-delta-gate.sh asserts in CI. So this is source-level tidiness, not a cost paid by shipped default artifacts.

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🤖 Filed with Claude Code.