feat(engine): theme zip importer with safety validation (Card theme phase 7)

Implements Phase 7 of CARD_PLAN.md — the entry point that takes a
user-supplied theme zip archive, validates it end-to-end, and
atomically unpacks it into the per-platform user themes directory.

Public API:
  import_theme(zip_path) -> Result<ThemeId, ImportError>
    Resolves user_theme_dir() and unpacks into <user>/<id>/.
  import_theme_into(zip_path, target_root) -> Result<ThemeId, ImportError>
    Test-friendly variant that takes the destination explicitly so
    unit tests never touch the global OnceLock override.

Safety guarantees enforced:
- 20 MB hard cap on archive size (read from the central directory
  before any extraction).
- Zip-slip path traversal rejected via ZipFile::enclosed_name plus a
  Component::Normal-only belt-and-braces check.
- Manifest parsed via ron::de and validated via the existing
  ThemeManifest::validate (Phase 2) — surfaces named diagnostics for
  missing-of-52, unknown keys, duplicate keys, and meta errors.
- Every referenced face + back rasterised through rasterize_svg as a
  structural validity check before any bytes hit the destination.
- Atomic install: writes to <root>/.<id>.tmp/ then std::fs::rename
  into place, with a recursive copy + remove fallback for cross-
  device renames. Failed extraction wipes the staging dir; the user
  themes root is never touched on error.
- Id collision with an existing theme dir rejected up front.

7 new tests covering the happy path plus six failure modes (missing
manifest, missing face, oversized archive, zip-slip, missing-file,
id collision). Tests build zips in tempfile::TempDir so they never
touch the real user themes directory.

Workspace deps: zip 8.6 (default-features off + deflate only),
tempfile 3.27 (dev only).

cargo check --workspace --all-targets / clippy --workspace
--all-targets -- -D warnings clean. cargo test could not be run in
this turn because cc disappeared from the sandbox; tests compile
under cargo check --tests and will run on a normal toolchain.

solitaire_engine/src/theme/mod.rs

@@ -12,6 +12,7 @@
 //! handles directly on card entities, so a theme switch propagates on
 //! the next frame without re-spawning anything.
 
+pub mod importer;
 pub mod loader;
 pub mod manifest;
 
@@ -25,6 +26,7 @@ use thiserror::Error;
 
 use solitaire_core::card::{Rank, Suit};
 
+pub use importer::{import_theme, import_theme_into, ImportError, ThemeId};
 pub use loader::{CardThemeLoader, CardThemeLoaderError};
 pub use manifest::ThemeManifest;