docs: factual README

[karelxfi]

Expands the README for the SQD Network worker node into a factual, plainly-written document.

What I confirmed (from the actual files)

• Purpose (Cargo.toml, src/main.rs, src/cli.rs, existing README.md): Rust implementation of the SQD Network worker node. Crate name sqd-worker, version 2.12.0, edition 2021. Downloads its assigned data-lake chunks from persistent storage (S3) and serves queries that reference those chunks. Replaces the previous Python version (subsquid/archive.py).
• P2P transport (src/main.rs, src/cli.rs): joins the network over a libp2p QUIC transport with a peer ID; mainnet/tethys networks have default boot nodes and assignment URLs filled in automatically.
• Query engine (Cargo.toml deps: polars, sqd-query, sqd-polars; src/main.rs sets POLARS_MAX_THREADS): queries run against local data using Polars and the SQD query crate.
• HTTP endpoints (src/http_server.rs): GET /worker/status, GET /worker/peer-id, GET /metrics (OpenMetrics text).
• CLI options (src/cli.rs): documented --data-dir, --prometheus-port (default 8000), --p2p-port/LISTEN_PORT (default 12345), --public-ip, --parallel-queries (20), --concurrent-downloads (3), --query-threads, --assignment-url.
• Build (rust-toolchain.toml = Rust 1.89; Dockerfile): cargo build --release produces target/release/sqd-worker; Docker build uses cargo-chef and needs protobuf-compiler, pkg-config, libssl-dev, libsqlite3-dev.
• License (Cargo.toml, LICENSE.md): AGPL-3.0-or-later.

Style

No marketing language, no em dashes, "onchain" kept as one word (not used in body), no "ship" verb. Only states what the source files show.

Links (all verified 200)

• https://docs.sqd.dev
• https://docs.sqd.dev/en/network
• https://docs.sqd.dev/subsquid-network/participate/worker/
• https://sqd.dev/network
• https://github.com/subsquid/sqd-network
• https://github.com/subsquid/data
• https://github.com/subsquid/archive.py/tree/master
• https://github.com/subsquid/specs/tree/main/network-rfc
• https://pola.rs

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