Fix hillSlope1 at 1: not identifiable with covariate coefficients

[eliotmcintire]

The spread link's slope, hillSlope1, was a DEoptim-fitted parameter (fireSenseUtils/R/objFunSpread.R, via estimateSpreadParams() in fireSense_SpreadFit) with bounds [0.2, 2], but it is not identifiable together with the covariate coefficients. With the linear predictor x = covariates %*% beta, hillSlope1 enters logistic3p()/logistic3pUpper() only as hillSlope1 * x, so scaling every coefficient by k and dividing hillSlope1 by k leaves every prediction unchanged, and DEoptim let coefficients drift along that ridge.

.objfunSpreadFit() (R/objFunSpread.R) now calls a new internal fixHillSlope1() on every par it receives, right after the yearSpreadSD element is stripped, reinserting hillSlope1 = 1 as the second logistic parameter before the parameters are split and evaluated. Callers (fireSense_SpreadFit) stop including hillSlope1 in DEoptim's lower/upper.

Verified with two new tests (test-fixHillSlope1.R): fixHillSlope1() inserts hillSlope1 = 1 at the right position for both named and unnamed input, and logisticAll() on the reconstructed vector gives the same prediction as calling it with hillSlope1 fixed at 1 by hand. test-yearSpreadSD.R is updated to expect the inserted hillSlope1 in the vector .objfunSpreadFit() hands to objFunInner(). Full suite, stacked on PR #92 (fix/youngage-exclusivity), scratch lib: 299 test blocks / 802 passed, 0 failed.

This branch is stacked on #92 (fix/youngage-exclusivity), so this PR's base is that branch, not development; it will retarget automatically once #92 merges. Version 0.2.3.9049. This pairs with PredictiveEcology/fireSense_spreadFit#47, similarly stacked, which needs this version.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CwcjqqK59FmTJscyi7xUqv

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.38%. Comparing base (b48ac45) to head (de6a21c).
⚠️ Report is 1 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development      #93      +/-   ##
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+ Coverage        44.35%   44.38%   +0.02%     
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  Files               40       40              
  Lines             3803     3805       +2     
===============================================
+ Hits              1687     1689       +2     
  Misses            2116     2116              

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