Fix mixed-precision stp description: half, not single

In mixed-precision mode (--mixed), stp is set to half_precision (kind=2),
not single_precision, as confirmed in src/common/m_precision_select.f90.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.claude/rules/fortran-conventions.md

@@ -33,7 +33,7 @@ These are caught by `./mfc.sh precheck` (source lint step 4/5):
 - `wp` (working precision): used for computation. Double by default.
 - `stp` (storage precision): used for I/O. Double by default.
 - In single-precision mode (`--single`): both become single.
-- In mixed-precision mode (`--mixed`): wp=double, stp=single.
+- In mixed-precision mode (`--mixed`): wp=double, stp=half.
 - MPI type matching: `mpi_p` must match `wp`, `mpi_io_p` must match `stp`.
 - Always use generic intrinsics: `sqrt` not `dsqrt`, `abs` not `dabs`.
 - Cast with `real(..., wp)` or `real(..., stp)`, never `dble(...)`.

CLAUDE.md

@@ -154,7 +154,7 @@ Changes to `src/common/` affect ALL three executables. Test comprehensively.
 ## Precision System
 
 - `wp` = working precision (computation). `stp` = storage precision (I/O).
-- Default: both double. Single mode: both single. Mixed: wp=double, stp=single.
+- Default: both double. Single mode: both single. Mixed: wp=double, stp=half.
 - MPI types must match: `mpi_p` ↔ `wp`, `mpi_io_p` ↔ `stp`.
 
 ## Code Review Priorities