gf180: add metal density fill rules (fill.json)

The gf180 platform had no metal density fill: it shipped FILL_CELLS
(std-cell filler) but no FILL_CONFIG, so OpenROAD's density_fill step
inserted nothing. gf180mcu's density deck enforces ">30% metal coverage
per layer, die-wide" (rules M2.4 / M3.4 / M5.4 / ...), which a sparse
design cannot meet without metal fill.

Add flow/platforms/gf180/fill.json (Metal1-Metal5) and point
FILL_CONFIG at it. Layer/datatype numbers match the platform's own
KLayout layer properties (Metal1 34/0 ... Metal5 81/0; fill geometry on
the *_Dummy datatype 4). Fill shape sizes use a 3-size pattern
[2.0, 1.0, 0.5] alternating H/V; Metal5 uses a tighter space_to_fill
(0.5) because the top routing layer carries the PDN stripes and leaves
less fillable area.

This is opt-in and does not change any existing result: USE_FILL still
defaults to 0, so density_fill remains a no-op unless a (chip/top-level)
design sets USE_FILL=1. Block / hard-macro builds keep USE_FILL=0 so
fill is inserted once, die-wide, at chip level rather than per block.

Verified with OpenROAD density_fill -rules on a routed gf180mcu design:
rules parse and fill inserts on Metal1-Metal5 with no errors.

Signed-off-by: Serge Rabyking <s.rabykin@gmail.com>

Author: lanserge

flow/platforms/gf180/config.mk

@@ -32,6 +32,10 @@ export FILL_CELLS                             ?= gf180mcu_fd_sc_mcu$(TRACK_OPTIO
                                                  gf180mcu_fd_sc_mcu$(TRACK_OPTION)$(POWER_OPTION)__fill_2 \
                                                  gf180mcu_fd_sc_mcu$(TRACK_OPTION)$(POWER_OPTION)__fill_1
 
+# Metal density fill rules (OpenROAD density_fill). Applied at chip level
+# only when USE_FILL=1; macro/block builds leave USE_FILL=0 (the default).
+export FILL_CONFIG                            ?= $(PLATFORM_DIR)/fill.json
+
 export TIE_CELL                               = gf180mcu_fd_sc_mcu$(TRACK_OPTION)$(POWER_OPTION)__filltie
 export ENDCAP_CELL                            = gf180mcu_fd_sc_mcu$(TRACK_OPTION)$(POWER_OPTION)__endcap
 export RC_FILE                                = $(PLATFORM_DIR)/setRC.tcl

flow/platforms/gf180/fill.json

@@ -0,0 +1,78 @@
+{
+    "layers" : {
+        "Metal1" : {
+            "layer":                   34,
+            "name":                    "Metal1",
+            "dir":                     "H",
+            "datatype":                 0,
+            "space_to_outline":         5,
+            "non-opc": {
+                "datatype":             4,
+                "width":                [2.00, 1.00, 0.5],
+                "height":               [2.00, 1.00, 0.5],
+                "space_to_fill":        1,
+                "space_to_non_fill":    1
+            }
+        },
+
+        "Metal2" : {
+            "layer":                   36,
+            "name":                    "Metal2",
+            "dir":                     "V",
+            "datatype":                 0,
+            "space_to_outline":         5,
+            "non-opc": {
+                "datatype":             4,
+                "width":                [2.00, 1.00, 0.5],
+                "height":               [2.00, 1.00, 0.5],
+                "space_to_fill":        1,
+                "space_to_non_fill":    1
+            }
+        },
+
+        "Metal3" : {
+            "layer":                   42,
+            "name":                    "Metal3",
+            "dir":                     "H",
+            "datatype":                 0,
+            "space_to_outline":         5,
+            "non-opc": {
+                "datatype":             4,
+                "width":                [2.00, 1.00, 0.5],
+                "height":               [2.00, 1.00, 0.5],
+                "space_to_fill":        1,
+                "space_to_non_fill":    1
+            }
+        },
+
+        "Metal4" : {
+            "layer":                   46,
+            "name":                    "Metal4",
+            "dir":                     "V",
+            "datatype":                 0,
+            "space_to_outline":         5,
+            "non-opc": {
+                "datatype":             4,
+                "width":                [2.00, 1.00, 0.5],
+                "height":               [2.00, 1.00, 0.5],
+                "space_to_fill":        1,
+                "space_to_non_fill":    1
+            }
+        },
+
+        "Metal5" : {
+            "layer":                   81,
+            "name":                    "Metal5",
+            "dir":                     "H",
+            "datatype":                 0,
+            "space_to_outline":         5,
+            "non-opc": {
+                "datatype":             4,
+                "width":                [2.00, 1.00, 0.5],
+                "height":               [2.00, 1.00, 0.5],
+                "space_to_fill":        0.5,
+                "space_to_non_fill":    0.5
+            }
+        }
+    }
+}