fix(webgpu): own mapped range backing stores (#36257)
## Summary
- back mapped WebGPU ranges with V8-owned `ArrayBuffer` storage
- copy writable ranges back before unmapping and detach every returned
range when unmapping or destroying the buffer
- cover multi-range writeback and mapped-range detachment
## Background
Writable mapped ranges previously used backend memory directly, while
readable ranges were copied without being retained for detachment. This
made returned `ArrayBuffer` lifetimes depend on the native mapping and
left readable ranges attached after `unmap()`.
Keeping JavaScript ranges in owned storage makes their lifetime
independent of the backend mapping. Writable data is copied back before
the mapping closes, including when callers request multiple
non-overlapping ranges.
## Tests
- `cargo test -p deno_webgpu`
- `cargo build -p deno -p test_server`
- `cargo test -p unit_tests --test unit webgpu_test`
- `cargo clippy -p deno_webgpu --all-targets -- -D warnings`
- `./tools/format.js ext/webgpu/buffer.rs tests/unit/webgpu_test.ts`
- `./tools/lint.js tests/unit/webgpu_test.ts`
