Fairsim partial counter configuration resets unspecified defaults

Expected Behavior

The partial counter configuration documented in tools/fairsim/README.md should override only the supplied values.

For example:

./fairsim \
  -counter-params=<(echo '{"CMS":{"W":100}}') \
  -- -tasks=1000 -keys=500

should set the CMS width to 100 while retaining all other values from counter.DefaultCounterParams.

Actual Behavior

fairsim unmarshals the JSON into a zero-valued counter.CounterParams. Omitted values such as MapLimit, CMS depth, growth settings, and reseeding interval therefore become zero.

The loaded configuration includes:

MapLimit: 0
CMS.W: 100
CMS.D: 0
CMS.Grow: zero values
CMS.Reseed.Interval: 0

With MapLimit set to zero, processing the first task can panic in mapCounter.updateHeap. It also means that changing one CMS setting unintentionally changes the rest of the simulated counter configuration.

Steps to Reproduce the Problem

1. Build fairsim from an unmodified checkout:

   make fairsim
   

2. Run the partial configuration example:

   ./fairsim \
     -counter-params=<(echo '{"CMS":{"W":100}}') \
     -- -tasks=1000 -keys=500
   

3. Observe that unspecified counter parameters are zero instead of retaining their defaults, potentially followed by an index-out-of-range panic.

Proposed Resolution

Initialize the configuration with counter.DefaultCounterParams before unmarshalling the supplied JSON. This makes partial configuration files behave as documented while continuing to support explicit zero values.

A regression test can load {"CMS":{"W":10}} and verify that only CMS.W changes while all other fields remain equal to counter.DefaultCounterParams.

Specifications

• Version: main at 632d694e7
• Platform: Linux amd64

Start with tools/fairsim/README.md and the fairsim configuration loading path, then run make fairsim and the partial CMS.W example. Compare the loaded values with counter.DefaultCounterParams and add the proposed regression test for {"CMS":{"W":10}}. Done means CMS.W is overridden while unspecified counter fields retain their defaults, including explicit zero values.