[Fix][WebGPU] Validate and bound symbolic stack allocations

[akaashrp]

Allow WebGPU code generation for local and workgroup allocations whose symbolic extents have finite compile-time upper bounds. WGSL requires array lengths to be compile-time constants, so codegen uses arithmetic analysis to derive and emit a static upper bound for expressions such as min(n, 64) (unbounded or nonpositive extents are rejected).

This PR:

• Detects element-count, byte-size, and aggregate workgroup-memory overflow
• Computes allocation sizes using WGSL array element stride
• Applies WebGPU's per-workgroup-variable 16-byte size rounding
• Tracks total workgroup storage used by each generated entry point
• Enforces max_shared_memory_per_block, corresponding to WebGPU's maxComputeWorkgroupStorageSize
• Sets the default WebGPU limit to 32 KiB to match TVM's JavaScript runtime contract while permitting target-specific overrides