Fix KSChan point-process OOB write of dparam[2]

[nrnhines]

Summary

• Removes a latent out-of-bounds write in KSChan::alloc that always did prop->dparam[2] = nullptr for point processes.
• That slot only exists when single-channel mode (or later ion/ligand fields) makes ppsize > 2. Plain point KSChan instances allocate only area and pnt, so index 2 was past the allocated Datum vector.
• Default ArrayPool freelist order hid the overflow inside the next free slab row; a reversed freelist exposes it under ASAN (see src/nrniv/kschan.cpp is allocator sensitive #3843).
• nrn_prop_datum_alloc already default-constructs every Datum to the null handle state, and dparam[0]/[1] are filled by point.cpp after nrn_alloc. The redundant nulling is removed; single_->alloc still runs when single_ is set and the singleptr slot is null.

Test plan

• Rebuild and run hoctests::test_kschan_py (with and without ASAN).
• Optionally re-apply the reverse freelist seed in arraypool.h from src/nrniv/kschan.cpp is allocator sensitive #3843 and confirm ASAN no longer fails on that test.
• Exercise single-channel paths in the same test (ks.single(1), etc.) to confirm singleptr allocation still works.

Fixes #3843

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

✔️ aa65940 -> artifacts URL

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.35%. Comparing base (2a21b48) to head (aa65940).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3844      +/-   ##
==========================================
+ Coverage   68.33%   68.35%   +0.02%     
==========================================
  Files         700      700              
  Lines      112198   112196       -2     
==========================================
+ Hits        76668    76692      +24     
+ Misses      35530    35504      -26     

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[azure-pipelines]

✔️ aa65940 -> Azure artifacts URL

[mgeplf]

LGTM, thanks.