Fix wasm32 build by gating MultiFileSession on non-wasm targets (#8612)

## Rationale for this change

The `MultiFileSession` holds a `moka` cache that reads
`std::time::Instant::now()` during construction. The `Instant` type is
unsupported on `wasm32` targets and panics with "time not implemented on
this platform". Since multi-file scanning is not available on wasm
anyway, this change gates the `MultiFileSession` registration behind a
`#[cfg(not(target_arch = "wasm32"))]` guard to allow the wasm32 build to
succeed.

## What changes are included in this PR?

Added a `#[cfg(not(target_arch = "wasm32"))]` attribute to the
`MultiFileSession` registration in `vortex/src/lib.rs` to prevent
instantiation of the session on wasm32 targets where
`std::time::Instant` is not available.

## What APIs are changed? Are there any user-facing changes?

No public APIs are changed. This is a build fix that prevents a panic on
wasm32 targets. Multi-file scanning functionality remains unavailable on
wasm, as it was before.

https://claude.ai/code/session_01P5VUBo8DhakxhhF1Ux2Kte

---------

Signed-off-by: Robert <robert@spiraldb.com>
Signed-off-by: Robert Kruszewski <github@robertk.io>
Co-authored-by: Claude <noreply@anthropic.com>

Author: robert3005

vortex/src/lib.rs

@@ -301,7 +301,11 @@ impl VortexSessionDefault for VortexSession {
             .with::<MemorySession>()
             .with::<RuntimeSession>();
 
-        #[cfg(feature = "files")]
+        // `MultiFileSession` holds a `moka` cache whose clock reads `std::time::Instant::now()`
+        // when constructed. `Instant` is unsupported on `wasm32` and panics with "time not
+        // implemented on this platform". Multi-file scanning is not available on wasm anyway, so
+        // only register this session variable on non-wasm targets.
+        #[cfg(all(feature = "files", not(target_arch = "wasm32")))]
         let session = {
             let session = session.with::<file::multi::MultiFileSession>();
             file::register_default_encodings(&session);