fix(rules): document the burst center-to-center convention as a deferred SRD approximation - the SRD (aiming-a-spell.md) measures a burst from a grid INTERSECTION with the far-edge rule ('count from intersection to intersection'; a square is inside only 'if the far edge of a square is within the spell's area'), but the engine's burst origin is a CELL (target_pos, centered_on_caster, resolved target) counted center-to-center 5-10-5 in _creatures_in_range; matching RAW exactly needs an intersection coordinate added to AreaEffect/Action.target_pos/the observation area snapshot contract plus a per-caster corner choice (the far-edge boundary result depends entirely on which of the cell's four corners is the origin), so it is DEFERRED with a precise explanation in areas.py and a pointer from _creatures_in_range; no behavior change, golden untouched

src/pf1e_simulator/areas.py

@@ -32,6 +32,37 @@ PF1e (CRB, Magic — Aiming a Spell):
 Bursts need no template here: they are circles around a center cell and are
 resolved by ``CombatEngine._area_creatures`` (which additionally drops
 creatures without line of effect from the burst's center).
+
+BURST MEASUREMENT - DOCUMENTED DEFERRED APPROXIMATION (cell vs intersection).
+The SRD ("Aiming a Spell - Area") measures a burst from a point of origin that
+is ALWAYS a grid intersection: "instead of counting from the center of one
+square to the center of the next, you count from intersection to intersection",
+and a square is inside the area only "if the far edge of a square is within the
+spell's area" (a square the area "only touches ... [at] the near edge" is
+unaffected). The engine cannot apply that rule faithfully because its burst
+origin is a CELL, not an intersection: ``Action.target_pos``, the
+``centered_on_caster`` caster square and the resolved target's square are all
+cells, and ``CombatEngine._creatures_in_range`` counts center-to-center 5-10-5
+(``grid.distance(center, cell) <= radius_cells``). Two consequences make a
+faithful fix out of scope for a contract-preserving pass:
+
+  1. A cell has four corner intersections, and which one is the point of origin
+     is a caster choice the engine never records. The far-edge rule's boundary
+     result (which diagonal squares are in vs out) depends entirely on that
+     choice, so there is no single "SRD-correct" square set to compute from a
+     cell center without first adding an intersection coordinate to the data
+     model.
+  2. Representing the origin as an intersection would change ``AreaEffect``,
+     ``Action.target_pos`` and the observation ``area`` snapshot contract (a
+     documented Phase 2 LLM frontier), and would rework the burst line-of-effect
+     and Reflex-cover origins (``{aim}`` / ``{center}`` cells).
+
+Until that representation change is made, bursts stay centered on the resolved
+cell and measured center-to-center under 5-10-5 - a symmetric, conservative
+approximation whose boundary squares can differ from RAW by one square on
+diagonals (e.g. a 15/20-ft burst's far diagonal cells). This is the residual
+recorded in docs/conformance-srd.md ("origin mesuré centre-à-centre plutôt
+qu'intersection-à-intersection").
 """
 
 from __future__ import annotations

src/pf1e_simulator/combat.py

@@ -7573,6 +7573,12 @@ class CombatEngine:
     ) -> list[CombatantState]:
         """Return creatures within radius_cells of center.
 
+        The burst center is a CELL and the radius is counted center-to-center
+        under 5-10-5 - a documented approximation of the SRD's
+        intersection-to-intersection far-edge measurement (see the
+        "BURST MEASUREMENT" note in ``areas.py`` for why a faithful
+        intersection origin is deferred).
+
         ``active_only`` (default) keeps only acting creatures; an area spell
         passes False so its burst also catches dying/unconscious creatures
         (SRD aiming-a-spell.md: a burst "affects whatever it catches in its