Rewrite in Rust: workspace + federation engine + CLI

[jeafreezy]

Summary

Complete rewrite of superstac as a Rust workspace. Replaces the Python prototype
with a federated STAC engine that queries multiple catalogs concurrently,
deduplicates results, and normalizes collection + asset names to canonical
forms. Ships both a library API and a superstac CLI.

Workspace

Crate	Purpose
superstac-core	domain models, errors, storage trait
superstac-config	YAML config loading
superstac-search	federated search logic
superstac-engine	runtime (health, introspection, orchestration)
superstac-cli	the superstac binary

What it does

• Federated search — concurrent fan-out, retry with exponential backoff,
  per-catalog timeouts, configurable concurrency cap.
• Collection + asset aliasing — declare per-catalog rename rules; users
  query canonical names; items come back normalized.
• Source selection — introspect /collections at startup; skip catalogs
  that can't serve the request.
• Deduplication — collapse items by id across catalogs with seen_in
  provenance.
• Discovery API — list_collections, catalogs_supporting,
  collections_by_catalog, describe_collection.
• CLI with search and collections subcommands, --json output,
  --verbose / --quiet log control.
• Tracing logging with RUST_LOG override.
• YAML config with shared + per-catalog settings.

Breaking change

Complete rewrite. The Python from superstac import ... API is gone.
Python bindings via PyO3 are on the roadmap.

Release prep

Unified workspace versioning (0.1.0), per-crate crates.io metadata,
CHANGELOG.md, cargo-release config. Ready for cargo publish once
crate names are claimed.

Test plan

• cargo build --workspace
• cargo test --workspace — 84 tests pass
• cargo doc --workspace --no-deps — no warnings
• cargo publish --dry-run -p superstac-core
• Manual: superstac collections against a real config
• Manual: superstac search -c sentinel-2-l2a -l 5

[jeafreezy]

Will be closing #18, #5, #10, #3 and #18