feat(reverse): isolate original physics substeps

Expose the original detail-derived integration count, active-ball count,
gravity, maximum speed, and movement flags in live traces. Allow probes to
force one physics substep so a single collision response can be observed
without the default three-step timer batch obscuring intermediate state.

Record the dynamically verified type-2 rail response: resolve velocity in the
registered line's normal/tangent basis, apply the two original Real48
coefficients, then advance from the previous position using that response.

Test Plan:
- `ruff check original/tools/trace_original_state.py original/tools/inject_original_state.py` -- passed
- one-substep probes against both vertical shooter walls -- passed
- ordinary rail responses matched 0.6 normal rebound and 0.1 tangent coupling
- `git diff --cached --check` -- passed

original/MECHANICS_PROGRESS.md

@@ -105,7 +105,8 @@ The executable image remains unchanged; only the current disposable Wine
 process is modified. Run the injector while the ball is waiting in the launcher
 so it can identify the live game object, or supply the previously reported
 `--object-base` to both the injector and tracer during a sequence of active
-probes.
+probes. `--substeps 1` isolates one collision integration step; omitting it
+retains the original configured detail level.
 
 A probe at `(289,94)` captured the original terminal-18 path. The collision
 suspended the ball while frames advanced from 10 through 18 at the configured
@@ -113,3 +114,11 @@ suspended the ball while frames advanced from 10 through 18 at the configured
 then cleared suspension during the same timer callback. The three default
 physics substeps each added 15 millipixels of vertical velocity, confirming the
 observed total gravity increment of 45 per timer tick.
+
+A one-substep probe against the vertical shooter walls recovered the type-2
+response arithmetic. For unit tangent `t` from the registered start to end and
+left normal `n`, an incoming contact with `vn = dot(v,n)` and `vt = dot(v,t)`
+uses `vn' = -0.6 * vn` and `vt' = vt + 0.1 * vn` for the ordinary rail
+coefficients stored in its two Real48 fields. The new position is the previous
+position plus this response velocity; the original does not separate a circle
+from the line with a modern capsule solver.

original/tools/inject_original_state.py

@@ -18,6 +18,7 @@ from trace_original_state import (
     NEXT_BALL_Y,
     OBJECT_BALL_X,
     OBJECT_BALL_Y,
+    PHYSICS_SUBSTEPS,
     PREVIOUS_BALL_X,
     PREVIOUS_BALL_Y,
     VELOCITY_X,
@@ -49,6 +50,12 @@ def parse_args() -> argparse.Namespace:
     parser.add_argument("--y", type=float, required=True, help="ball center y in logical pixels")
     parser.add_argument("--vx", type=float, default=0.0, help="x velocity in pixels per timer tick")
     parser.add_argument("--vy", type=float, default=0.0, help="y velocity in pixels per timer tick")
+    parser.add_argument(
+        "--substeps",
+        type=int,
+        choices=range(1, 6),
+        help="temporarily force the original physics substep count",
+    )
     return parser.parse_args()
 
 
@@ -106,6 +113,8 @@ def main() -> None:
                 (NEXT_BALL_Y, next_y),
             ):
                 memory.write(data_base + offset, value)
+            if args.substeps is not None:
+                memory.write(data_base + PHYSICS_SUBSTEPS, struct.pack("<H", args.substeps))
             for offset, value in (
                 (VELOCITY_X, velocity_x),
                 (VELOCITY_Y, velocity_y),

original/tools/trace_original_state.py

@@ -31,8 +31,12 @@ CLAW_FRAME = 0x086D
 CLAW_TARGET_FRAME = 0x086F
 CLAW_SPRITE_BANK = 0x0873
 CLAW_ANIMATION_BLOCKED = 0x0874
+PHYSICS_SUBSTEPS = 0x0871
 
 # Recovered offsets in the live game-window object.
+MAXIMUM_SPEED = 0x0056
+GRAVITY_PER_SUBSTEP = 0x005A
+ACTIVE_BALL_COUNT = 0x0061
 VELOCITY_X = 0x0BAA
 VELOCITY_Y = 0x0BAE
 PREVIOUS_BALL_X = 0x0BBA
@@ -44,6 +48,8 @@ RIGHT_FLIPPER_POSITION = 0x0BD6
 LEFT_FLIPPER_DOWN = 0x0BDA
 RIGHT_FLIPPER_DOWN = 0x0BDB
 BALL_SUSPENDED = 0x0BDF
+BALL_ACTIVE = 0x0BDD
+COLLISION_DISABLED = 0x0BD8
 OBJECT_READ_SIZE = 0x0BE0
 
 
@@ -208,6 +214,10 @@ CSV_FIELDS = [
     "elapsed_us",
     "player_count",
     "current_player",
+    "physics_substeps",
+    "active_ball_count",
+    "maximum_speed_milli_per_tick",
+    "gravity_milli_per_substep",
     "ball_x_milli",
     "ball_y_milli",
     "next_ball_x_milli",
@@ -222,6 +232,8 @@ CSV_FIELDS = [
     "right_flipper_position",
     "left_flipper_down",
     "right_flipper_down",
+    "ball_active",
+    "collision_disabled",
     "ball_suspended",
     "claw_active",
     "claw_frame",
@@ -235,6 +247,10 @@ def sample_state(data: bytes, obj: bytes) -> tuple[int, ...]:
     return (
         data[PLAYER_COUNT],
         data[CURRENT_PLAYER],
+        unpack_u16(data, PHYSICS_SUBSTEPS),
+        obj[ACTIVE_BALL_COUNT],
+        unpack_i32(obj, MAXIMUM_SPEED),
+        unpack_i32(obj, GRAVITY_PER_SUBSTEP),
         unpack_i32(data, BALL_X),
         unpack_i32(data, BALL_Y),
         unpack_i32(data, NEXT_BALL_X),
@@ -249,6 +265,8 @@ def sample_state(data: bytes, obj: bytes) -> tuple[int, ...]:
         unpack_u16(obj, RIGHT_FLIPPER_POSITION),
         obj[LEFT_FLIPPER_DOWN],
         obj[RIGHT_FLIPPER_DOWN],
+        obj[BALL_ACTIVE],
+        obj[COLLISION_DISABLED],
         obj[BALL_SUSPENDED],
         data[CLAW_ACTIVE],
         unpack_u16(data, CLAW_FRAME),