Client::join API

[texodus]

This PR adds a reactive Client::join to JavaScript, Python and Rust.

• Adds Client::join API across all platforms (Rust, JavaScript, Python) to create read-only tables by joining two source tables on a shared column (potentially with different names, but they must have the same type).
• Supports inner (default), left, and outer join types, with an optional right_on parameter for joining on differently-named columns. Duplicates (cross-product) are handled correctly and left-precedence natural order is preserved for visual stability.
• Joined Table objects are reactive — updates to either sourceTable automatically recompute the join and propagate to any derived View.
• Joins can be chained and will preserve evaluation and deletion order.
• and a new set of benchmarks (which are only useful going forward since no prior version of Perspective supports this API).
• Adds docs for join types, options, and reactivity; how-to guides for JS, Python, and Rust.
• Adds comprehensive test suites for all join types, indexed tables, and right_on behavior.

const left = await worker.table(data);
const right = await worker.table(other_data);
const options = {join_type: "outer", name: "Joined Table"};
const joined_table = await worker.join(left, right, "Row ID", options);