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>

README.md

@@ -916,9 +916,12 @@ Règles modélisées :
   largeur valant la longueur à chaque distance) et la **ligne** (5 ft de large,
   longueur = `radius_ft`) partent de la **case du jeteur** et sont pointés
   vers la cible résolue (`target_id`) ou le point désigné (`target_pos`) — le
-  cône exclut la case d'origine, la ligne commence dans la case du jeteur
+  cône exclut la case d'origine, la ligne part d'un **coin** de la case du
-  (« the bolt begins at your fingertips », SRD Lightning Bolt : le jeteur est
+  jeteur et n'affecte **pas** sa propre case (CRB « Aiming a Spell » : « It
-  donc dans sa propre ligne). Toute créature dont une case occupée intersecte
+  starts from any corner of your square » — SRD Lightning Bolt : « The bolt
+  begins at your fingertips », le trait quitte la case du jeteur au lieu de la
+  remplir ; un viseur dégénéré origine==cible n'affecte personne). Toute
+  créature dont une case occupée intersecte
   le gabarit subit l'effet, avec sauvegarde et RS par créature comme pour un
   burst (application partagée, géométrie dans `areas.py`). Le snapshot
   d'observation expose `{shape, radius_ft}` via la clé `area`.

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
+  line's length. The line "starts from any corner of your square", so the
-  "begins at your fingertips", SRD Lightning Bolt).
+  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
+    its range"). The line "starts from any corner of your square" (CRB, Magic
-    cells simply drop out of the template (creatures only exist in bounds).
+    — Aiming a Spell), so the CASTER's own square is NOT affected: the
-    A degenerate aim (origin == aim) affects only the origin cell.
+    ``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()
+        return frozenset()
-    # Extend the aim vector so the ray spans `length_cells` squares (the
+    # Extend the aim vector so the ray reaches `length_cells` squares BEYOND
-    # dominant axis step count of a Bresenham line is max(|d_row|, |d_col|)).
+    # the origin (the dominant-axis step count of a Bresenham line is
-    scale = length_cells / max(abs(d_row), abs(d_col))
+    # 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 line is `length_cells` squares LONG starting just past the caster:
-    # the first of them (120 ft / 5 ft = 24 squares for Lightning Bolt).
+    # 120 ft / 5 ft = 24 squares, none of them the caster's own (SRD Lightning
-    return frozenset(_bresenham(origin, end)[:length_cells])
+    # 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]:

tests/test_area_shapes.py

@@ -197,8 +197,9 @@ def test_line_hits_the_straight_ray_only() -> None:
     """Given: caster (6,2) aiming east at foe_ray (6,5); foe_off (5,5) sits a
     row away, foe_west (6,0) is behind the caster
     When: cast the 120-ft line aimed at foe_ray
-    Then: every creature ON the ray (the caster's own cell — the line starts
+    Then: every creature ON the ray beyond the caster's own square (foe_ray)
-    at your fingertips — and foe_ray) takes damage; the off-row and the
+    takes damage; the caster's own cell is NOT part of the line (CRB: it
+    "starts from any corner of your square"), and the off-row and the
     behind-caster foes are outside the 5-ft-wide template."""
     caster = _caster()
     foe_ray = make_state(make_combatant("foe_ray", hp=100), "monsters", (6, 5))
@@ -210,7 +211,7 @@ def test_line_hits_the_straight_ray_only() -> None:
     )
     assert result.ok
     assert foe_ray.hp == 100 - 4
-    assert caster.hp == 100 - 4  # the line's first square is the caster's own
+    assert caster.hp == 100  # the caster's own square is outside the line
     assert foe_off.hp == 100
     assert foe_west.hp == 100
 

tests/test_spells.py

@@ -462,22 +462,24 @@ class TestLightningBoltSave:
     1d6 per caster level max 10d6, ref save for half) cast by the level-5
     fixture at a target straight ahead
     When: the primary target succeeds its save
-    Then: damage is halved — and the caster, whose own square the line
+    Then: damage is halved — and the caster is NOT caught by their own line
-    starts in ("the bolt begins at your fingertips"), is caught as well."""
+    (CRB, Magic — Aiming a Spell: the line "starts from any corner of your
+    square", so the caster's own square is outside the template)."""
 
     def test_successful_save_still_halves(self) -> None:
         caster = _make_caster()
         target = _make_target(hp_max=200)
-        # primary save (target) + secondary save (caster, same line) +
+        # primary save (target, SAVED on a natural 20) + 5 damage dice
-        # 5 damage dice each (1d6/level, level 5)
+        # (1d6/level, level 5); the caster is no longer in the line so no
+        # secondary save and no caster damage dice are rolled.
         engine, c, t = _make_engine(
-            caster, target, [20, 20] + [3] * 10
+            caster, target, [20] + [3] * 5
         )
         hp_before = t.hp
         engine._cast_spell(c, t, "Lightning Bolt")
         full = 3 + 3 + 3 + 3 + 3
         assert t.hp == hp_before - (full // 2)
-        assert c.hp == 20 - (full // 2)
+        assert c.hp == 20  # the caster's own square is outside the line
 
 
 class TestDamageEffectHalfOnSaveDefault: