Fix incorrect and misleading comments across src/

[BakaOverflow]

Description

Fixes seven incorrect or misleading comments across src/, as listed in #1497. All changes are comment-only — no executable code, logic, or formatting was touched.

1. m_helper.fpp (double_factorial, factorial): selected_int_kind(18) requests 18 decimal digits, not 18 bytes — corrected both.
2. m_collisions.fpp: module @brief was copy-pasted from an IBM module; rewrote it to describe the module's actual collision-force handling.
3. m_data_output.fpp: Rc_min tracks a minimum (minval, Rc_min_glb < Rc_min), so "maximum" → "minimum".
4. m_derived_types.fpp: p0/m0 are bubble pressure/mass; comments said size/velocity (copied from R0/V0).
5. m_riemann_solver_hllc.fpp: removed a duplicate "HLLC".
6. m_precision_select.f90: clarified half_precision is a hardcoded kind (its siblings are computed via selected_real_kind).
7. m_mpi_common.fpp: fixed a dangling "and."; removed a duplicate s_mpi_reduce_maxloc summary line.

Note: the issue cited m_riemann_solvers.fpp:1779 for item 5, but that module was split into per-scheme files after the issue was filed, so the fix lands in m_riemann_solver_hllc.fpp. A few other line numbers had also drifted and were re-verified against current master.

Closes #1497.

Type of change (delete unused ones)

• Documentation

Testing

Comment-only change with no runtime behavior, so there's nothing to unit-test. Validated that the diff is comment-only and the project's checks pass: ./mfc.sh format (no reformatting), ./mfc.sh lint (ruff + 349 toolchain tests passed), and the pre-commit precheck (same as the CI lint-gate) passed all seven gates (formatting, spell check, toolchain lint, source lint, doc references, parameter docs, example cases).

Checklist

• I added or updated tests for new behavior (N/A — comment-only, no behavior change)
• I updated documentation if user-facing behavior changed (these fixes are the documentation)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.39%. Comparing base (9906fa6) to head (07ebfba).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1624   +/-   ##
=======================================
  Coverage   60.39%   60.39%           
=======================================
  Files          83       83           
  Lines       19854    19854           
  Branches     2955     2955           
=======================================
  Hits        11990    11990           
  Misses       5863     5863           
  Partials     2001     2001           

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