Add --fixed-diff option for upstream share filtering

[danielj86]

Summary

Implements upstream difficulty filtering to prevent share rejections when pool VarDiff exceeds worker difficulty.

Problem

When operating a proxy with fixed low-difficulty workers connecting to a VarDiff-enabled pool:

1. Workers submit shares at their assigned difficulty (e.g., 50,000)
2. Pool VarDiff algorithm increases difficulty over time (e.g., to 5,000,000)
3. Proxy forwards all worker shares that pass worker-level validation
4. Pool rejects shares below its current difficulty threshold
5. Results in mass "low difficulty share" rejections and wasted bandwidth

Solution

New --fixed-diff=N configuration option that:

• Sets a minimum difficulty threshold for upstream pool submission
• Filters shares below this threshold before forwarding to pool
• Returns "OK" to workers for filtered shares (transparent operation)
• Preserves donation share flow (filtering only applies to user pool)

Usage

CLI:

./xmrig-proxy -o pool:3333 -u wallet --custom-diff=50000 --fixed-diff=500000

JSON Config:

{
    "custom-diff": 50000,
    "fixed-diff": 500000
}

Behavior

Setting	Effect
--custom-diff=50000	Workers receive 50k difficulty jobs
--fixed-diff=500000	Only shares ≥500k forwarded to pool

• Workers receive "OK" for all valid shares (including filtered ones)
• Pool only receives high-difficulty shares
• No VarDiff ramp-up from low-diff share flood
• Zero "low difficulty share" rejections

Implementation Details

• Validation: Minimum 100, maximum INT_MAX
• Filtering location: SimpleMapper::submit() after strategy selection
• Donation bypass: Filtering skipped when donate strategy is active
• Logging: Filtered shares logged at DEBUG level only

Files Changed

File	Change
src/base/kernel/interfaces/IConfig.h	FixedDiffKey = 1118
src/core/config/Config.h	Member variable and accessor
src/core/config/Config.cpp	Parser and serializer
src/core/config/Config_platform.h	CLI option registration
src/core/config/ConfigTransform.cpp	CLI-to-JSON transform
src/core/config/usage.h	Help text
src/proxy/splitters/simple/SimpleMapper.cpp	Core filtering logic
src/Summary.cpp	Startup configuration display

Test Plan

• Verify --fixed-diff appears in --help output
• Confirm startup displays fixed-diff value when configured
• Test share filtering with DEBUG logging enabled
• Validate workers receive "OK" for filtered shares
• Confirm pool receives only shares meeting threshold