DNN: Add large scalar fastconv kernel for WASM/no-SIMD build

[fanchenkong1]

Summary

This PR adds a larger scalar kernel (convBlockNoSIMD4x24) in the DNN fastConv path. It targets non-SIMD WebAssembly build.

Background

In OpenCV.js/no-SIMD builds, OpenCV.js 4.x shows ~35% slowdown compared to 4.5.2 on Intel Core Ultra 7 258V. The regression comes from using a small scalar fallback, which increases the memory load/store pressure.

What's changed

• Add 4x24 scalar micro-kernel (convBlockNoSIMD4x24) for WASM no-SIMD path, other shapes keep on existing convBlockNoSIMD code.
• Kernel processes 2 rows at a time to avoid store-bound.

Performance

Workload: Face detection and image classification using res10_300x300_ssd_iter_140000_fp16.caffemodel and squeezenet.onnx v 1.0 on five images (720x480)
Environment: WASM no-SIMD build/Chrome stable 138/on Intel Core Ultra 7 258V

	OpenCV.js 4.5.2 (baseline)	OpenCV.js 4.x	OpenCV.js 4.x + this PR
Diff(execution time)	0%	+35%	-19%

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[fanchenkong1]

Ubuntu2004-x64-OpenVINO bot reports a timeout failure at Accuracy:gapi.

The action 'Accuracy:gapi' has timed out after 60 minutes.

This PR only touches the WASM build path. May I re-run the trybot to check if it was a temporary issue?

[fanchenkong1]

Ubuntu2004-x64-OpenVINO bot reports a timeout failure at Accuracy:gapi.

The action 'Accuracy:gapi' has timed out after 60 minutes.

This PR only touches the WASM build path. May I re-run the trybot to check if it was a temporary issue?

@vpisarev , could you please take a look? Thanks!

[fanchenkong1]

Ubuntu2004-x64-OpenVINO bot reports a timeout failure at Accuracy:gapi.

The action 'Accuracy:gapi' has timed out after 60 minutes.

This PR only touches the WASM build path. May I re-run the trybot to check if it was a temporary issue?
@asmorkalov, could you please take a look? Thanks!