Update ARCHITECTURE.md to reflect current implementation

[jhamman]

Summary

• HTTP storage: Documented SimpleHttpStore (CDN-compatible, tolerates missing Content-Length) replacing references to zarrs_http::HTTPStore
• Consolidated metadata: Moved from Future Work to its own implemented section; documented both V2 .zmetadata and V3 inline consolidated_metadata support, plus the zarrsJlStorageGet FFI function
• Cargo features: Updated to match actual Cargo.toml (only icechunk is feature-gated; filesystem/HTTP/S3/GCS are unconditional)
• Storage dispatch: Updated code snippet to show current URL pipeline implementation
• S3/GCS: Moved from Future Work to implemented; only Azure remains as future
• FFI description: Updated to reflect that zarrs_jl depends on zarrs directly, not zarrs_ffi
• Package structure: Fixed Rust source files (lib.rs, http_store.rs, icechunk.rs), added url_pipeline.jl and new test files

Closes #2

Test plan

• Verify all code snippets match actual source files
• Confirm Future Work section only lists genuinely unimplemented features
• Review package structure listing against actual file tree

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