Add comments to TrimSP Fortran core

fortran/trimspNL.F

@@ -401,7 +401,9 @@ C     LMAX is maximum number of layers and JMAX is maximum number of
 C     elements per layer.
       JMAX=5
 
-C     This part reads the input file (new format) 
+C     This part reads the input file (new format).
+C     The JavaScript/Electron frontend writes this sequential block layout,
+C     so the READ order below must stay in sync with CreateInpFile().
       OPEN(UNIT=99,file=errnam,STATUS='replace')
       OPEN(UNIT=11,file=innam,STATUS='unknown',ERR=1359)
 C     First line: properties of projectile
@@ -814,6 +816,9 @@ C
       ENDDO
 C
 C     PROJECTILE LOOP
+C     Each pass transports one projectile history. The state vectors
+C     X/Y/Z, E, direction cosines, current layer index, etc. are updated
+C     in-place until the particle stops, backscatters or transmits.
 C
  1    CONTINUE
       NPROJ=NPROJ+1
@@ -829,6 +834,10 @@ C
       KK1=KK0
 C
 C     COLLISION LOOP (INCLUDES WEAK SIMULTANEOUS COLL. FOR KK1.LT.4)
+C     KK controls the treatment of weak simultaneous collisions.
+C     For every active projectile we choose a collision partner, sample an
+C     impact parameter, solve the scattering geometry for the selected
+C     interatomic potential and then update the projectile state.
 C
       DO 245 KK=KK1,0,-1
 C
@@ -3374,6 +3383,10 @@ C     &     I0,3x))
      &     ,1x))
 
       inquire(FILE='fort.33',EXIST=FORT33)
+C     fort.33 is the compact per-run summary consumed by the GUI for scan
+C     plots (fractions stopped in each layer, backscattering, transmission,
+C     mean depth, etc.). The header line is written once, then each run
+C     appends a single summary row.
       if (.not.FORT33) then
          open(33)
          WRITE(33,7802)  (chem(k),k=1,NLayers)