fix(examples): use pod_collect_to_vec for mapped buffer reads

[countgitmick]

Summary

• Replace bytemuck::cast_slice with bytemuck::allocation::pod_collect_to_vec when reading from mapped buffer ranges
• Add extern_crate_alloc feature to workspace bytemuck dependency

cast_slice requires the source slice to be properly aligned for the target type, but BufferView from get_mapped_range() is not guaranteed to meet alignment requirements. pod_collect_to_vec copies bytes into a new properly-aligned Vec, avoiding potential undefined behavior from misaligned reads.

As suggested by @cwfitzgerald in #6191.

Files changed

• examples/src/hello_workgroups/mod.rs
• examples/src/hello_synchronization/mod.rs
• examples/src/hello_compute/mod.rs
• examples/src/repeated_compute/mod.rs
• examples/src/timestamp_queries/mod.rs
• Cargo.toml (add extern_crate_alloc feature to bytemuck)

Test plan

• cargo check -p wgpu-examples
• cargo fmt --check -p wgpu-examples

Closes #6191

[countgitmick]

Rebased on trunk to resolve merge conflicts from the examples restructure. Also caught a missed call site in big_compute_buffers/mod.rs and fixed cooperative_matrix/mod.rs which has the same pattern.

Updated file list (8 files):

• examples/features/src/hello_workgroups/mod.rs
• examples/features/src/hello_synchronization/mod.rs
• examples/features/src/repeated_compute/mod.rs
• examples/features/src/timestamp_queries/mod.rs
• examples/features/src/big_compute_buffers/mod.rs (newly caught)
• examples/features/src/cooperative_matrix/mod.rs (newly caught)
• examples/standalone/01_hello_compute/src/main.rs
• examples/standalone/01_hello_compute/Cargo.toml

[ErichDonGubler]

question: Is there an interesting CHANGELOG entry we might produce for this? If this is obscure enough, I don't feel like we need to chase that down.

[cwfitzgerald]

It's example only.