Fix grid stretching using wrong array bounds for y_cc and z_cc

[sbryngelson]

Summary

Severity: CRITICAL — corrupts y/z cell-center coordinates when grid stretching is active.

File: src/pre_process/m_grid.f90, lines 134 and 171

After applying grid stretching, the cell-center arrays are computed with the wrong left-hand side bounds: y_cc(0:m) and z_cc(0:m) use the x-dimension size m instead of the y-dimension n and z-dimension p.

Before

! y-direction (line 134)
y_cc(0:m) = (y_cb(0:n) + y_cb(-1:n-1))/2._wp   ! m is x-size, should be n

! z-direction (line 171)
z_cc(0:m) = (z_cb(0:p) + z_cb(-1:p-1))/2._wp   ! m is x-size, should be p

After

y_cc(0:n) = (y_cb(0:n) + y_cb(-1:n-1))/2._wp
z_cc(0:p) = (z_cb(0:p) + z_cb(-1:p-1))/2._wp

Why this went undetected

When m == n == p (equal resolution in all directions), the wrong index produces the same result. The bug only manifests with anisotropic grids and stretching enabled.

Test plan

• Run 3D case with grid stretching and anisotropic resolution (m != n != p)
• Verify y_cc and z_cc cell centers match expected stretched coordinates

🤖 Generated with Claude Code

Fixes #1200

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Nitpicks 🔍

🔒 No security issues identified

⚡ Recommended areas for review Array bounds correction The PR fixes y/z cell-center computation by using the correct dimension sizes for the slices used to compute cell centers. Verify that the arrays have the expected lower bounds (including the -1 boundary) and sizes so the slices (0:n) and (-1:n-1) / (0:p) and (-1:p-1) are valid for all build configurations and array declarations. Also validate behavior for degenerate sizes (n==0 or p==0) and MPI-parallel cases. In-place scaling side-effect The code divides y_a, y_b, z_a, and z_b in-place by length. That alters the original parameter values for the rest of the routine/module. Confirm this side-effect is intended; if not, use temporaries to avoid mutating the original scaling parameters.

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Pull request overview

Fixes incorrect array section bounds used when computing cell-center coordinates (y_cc, z_cc) during grid stretching, which corrupted coordinates for anisotropic grids.

Changes:

• Use n (y dimension) instead of m when assigning y_cc(0:...).
• Use p (z dimension) instead of m when assigning z_cc(0:...).

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.05%. Comparing base (84c46e0) to head (b95f40c).

Files with missing lines	Patch %	Lines
src/pre_process/m_grid.f90	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1179   +/-   ##
=======================================
  Coverage   44.05%   44.05%           
=======================================
  Files          70       70           
  Lines       20498    20498           
  Branches     1990     1990           
=======================================
  Hits         9030     9030           
  Misses      10329    10329           
  Partials     1139     1139           

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