Add shared provider icon store

[konojunya]

Summary

• release the pre-1.0 CLI as 0.3.0 with a provider-first import contract: stack icons import <provider> --accept-terms
• download pinned official archives over HTTPS, bound response bodies, and verify full SHA-256 values before ZIP processing
• store packs under the XDG user config directory by default, with default_icons_path, -o, and --provider-pack overrides
• discover all known provider child directories from one icon-store root while ignoring unknown directories
• update provider documentation, dependency attribution, and integration coverage for shared and project-local stores

Verification

• cargo test --locked
• STACK_SPECIFICATION_DIR=../specification cargo test --features conformance --test formatter-conformance --locked
• cargo clippy --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --locked
• STACK_SPECIFICATION_DIR=../specification cargo llvm-cov --all-features --locked --fail-under-lines 90 --fail-under-functions 95 --fail-under-regions 90
• cargo build --release --locked
• node scripts/validate-provider-catalogs.mjs
• python3 scripts/validate-render.py
• live simple-icons import and default-store render smoke test