readonly mappedfile wrapper class, use_mapped_model_loading option

[nihui]

z-image-ncnn 1024x1024

end2end	9060xt	7900xtx
baseline	1m7.296s	38.35s
+mmap	58.124s	31.86s

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.06%. Comparing base (7d5d32d) to head (b5e3047).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/net.cpp	80.00%	3 Missing ⚠️
src/c_api.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6537      +/-   ##
==========================================
+ Coverage   92.94%   93.06%   +0.11%     
==========================================
  Files         809      847      +38     
  Lines      256714   266763   +10049     
==========================================
+ Hits       238613   248268    +9655     
- Misses      18101    18495     +394     

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[Copilot]

Pull request overview

This PR introduces an optional memory-mapped (“mmap”) path for loading model weights to reduce load time and memory copying, along with a new Option::use_mapped_model_loading toggle and updated in-memory load APIs to return size_t bytes consumed.

Changes:

• Add MappedFile wrapper and a use_mapped_model_loading Option to enable mapped model loading on supported platforms.
• Switch Net::load_param(const unsigned char*) / Net::load_model(const unsigned char*) return types from int to size_t.
• Update the squeezenet test configuration and docs to use/reflect the new option and signatures.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
tests/test_squeezenet.cpp	Enables mapped model loading and related options for one benchmark/test configuration.
src/platform.h.in	Adds cross-platform MappedFile implementation and required includes for mmap/Windows mapping.
src/option.h	Adds use_mapped_model_loading and adjusts option field layout/types.
src/option.cpp	Initializes the newly added Option fields.
src/net.h	Updates in-memory load APIs to return size_t.
src/net.cpp	Implements mapped-file loading path in Net::load_model(path) and updates in-memory load implementations.
docs/how-to-use-and-FAQ/ncnn-load-model.md	Updates documentation to reflect size_t return types for in-memory loading.

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