fix(rules): exclude the caster from a line spell's area

CRB Magic - Aiming a Spell: a line 'starts from any corner of your square', so the caster's own square is outside the template (SRD Lightning Bolt: 'The bolt begins at your fingertips' - it leaves the square, it does not fill it). line_cells now drops the origin and takes the length_cells squares beyond it; a degenerate aim affects nothing. Updated the Lightning Bolt and Line Sparks tests (caster no longer caught) and the README.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/pf1e_simulator/areas.py

@@ -11,8 +11,11 @@ PF1e (CRB, Magic — Aiming a Spell):
   limit of its range [...]. A line-shaped spell affects all creatures in
   squares through which the line passes." — modeled here as the 5-ft-wide
   Bresenham ray from the origin through the aim cell, truncated to the
-  line's length (the origin cell counts as the first square: the bolt
-  "begins at your fingertips", SRD Lightning Bolt).
+  line's length. The line "starts from any corner of your square", so the
+  CASTER's own square is NOT one of the affected squares: the affected
+  squares are the ones beyond the origin along the ray (SRD Lightning Bolt:
+  "The bolt begins at your fingertips" — it leaves your square, it does not
+  fill it).
 
 Bursts need no template here: they are circles around a center cell and are
 resolved by ``CombatEngine._creatures_in_range`` (unchanged).
@@ -74,23 +77,28 @@ def line_cells(
 
     The line runs along the Bresenham ray from the origin toward ``aim``,
     extended to ``length_cells`` squares (CRB: it "extends to the limit of
-    its range"). The origin cell is the first square of the line. Out-of-bounds
-    cells simply drop out of the template (creatures only exist in bounds).
-    A degenerate aim (origin == aim) affects only the origin cell.
+    its range"). The line "starts from any corner of your square" (CRB, Magic
+    — Aiming a Spell), so the CASTER's own square is NOT affected: the
+    ``length_cells`` affected squares are the ones *beyond* the origin along
+    the ray. Out-of-bounds cells simply drop out of the template (creatures
+    only exist in bounds). A degenerate aim (origin == aim) affects nothing.
     """
     if length_cells <= 0:
         return frozenset()
     d_row = aim[0] - origin[0]
     d_col = aim[1] - origin[1]
     if d_row == 0 and d_col == 0:
-        return frozenset({origin}) if grid.in_bounds(origin) else frozenset()
-    # Extend the aim vector so the ray spans `length_cells` squares (the
-    # dominant axis step count of a Bresenham line is max(|d_row|, |d_col|)).
-    scale = length_cells / max(abs(d_row), abs(d_col))
+        return frozenset()
+    # Extend the aim vector so the ray reaches `length_cells` squares BEYOND
+    # the origin (the dominant-axis step count of a Bresenham line is
+    # max(|d_row|, |d_col|)); the +1 accounts for the origin square we drop.
+    scale = (length_cells + 1) / max(abs(d_row), abs(d_col))
     end = (origin[0] + round(d_row * scale), origin[1] + round(d_col * scale))
-    # The line is `length_cells` squares LONG: the origin square counts as
-    # the first of them (120 ft / 5 ft = 24 squares for Lightning Bolt).
-    return frozenset(_bresenham(origin, end)[:length_cells])
+    # The line is `length_cells` squares LONG starting just past the caster:
+    # 120 ft / 5 ft = 24 squares, none of them the caster's own (SRD Lightning
+    # Bolt: "The bolt begins at your fingertips" — it leaves your square).
+    ray = _bresenham(origin, end)
+    return frozenset(cell for cell in ray[1 : length_cells + 1] if grid.in_bounds(cell))
 
 
 def _bresenham(start: Pos, end: Pos) -> list[Pos]: