Split web/ into senbei-wasm crate + static assets

The Rust bindings move from web/src to a top-level senbei-wasm crate (still
outside the workspace, own Cargo.lock), matching the other senbei-* crates.
web/ keeps only the static frontend; wasm-pack emits the JS/wasm package into
web/pkg/ via --out-dir. JS glue renamed senbei_web -> senbei_wasm with the
crate.

.gitignore

@@ -153,6 +153,8 @@ target/
 !/samples/README.md
 
 ### senbei web build ###
+# The wasm crate builds out-of-workspace in senbei-wasm/ and wasm-pack emits
+# the JS/wasm package into web/pkg/ for the static frontend.
+/senbei-wasm/target/
 /web/pkg/
-/web/target/
 /web/.playwright-cli