Restack to four signal layers and route the board

The Ti375 fanout could not be resolved with three routing layers, so L5
stops being a dedicated 1.8-V plane and becomes a signal layer.  No rail
owns a whole layer any more: the supplies are split into zone islands on
L4 and L6, L2 and L7 stay solid and completely unrouted, and F.Cu, L3, L5
and B.Cu are all available for signals.

Routing is staged rather than handed to the autorouter wholesale.  The
BGA escape, the buck high-current loops, plane stitching and the local
power completion all run first and are locked; Freerouting then sees only
the ordinary digital classes, with everything else present as fixed
copper.  finalize_routing.py installs the audited RF and USB geometry and
closes whatever the autorouter left.

Four bugs behind the routing failures, all of them the same shape - a
layer list written down instead of read from the board, which was correct
while L5 was a plane and silently wrong afterwards:

- export_routing_stage.py emitted via keepouts for three layers only, so
  every existing via was invisible to the router on L5.  It drove traces
  straight through them: 102 shorting_items in DRC.
- route_local_power.py never used L5 at all, and picked up escape vias on
  two layers instead of all four.
- add_rf_ground() judged fence positions against vias and pads but not
  tracks.  It runs after the session import, when that region already has
  routed copper, and dropped a ground via on top of a signal trace.

route_local_power.py also grew a via cloud on every rail: reaching a
plane counted as progress even when it merged nothing, so the same pad
collected a via per pass, 0.4 mm further out each time.  Progress is now
measured by the group count actually falling, and groups already sitting
on their plane are skipped.  That removed 404 redundant vias.

The MAX2771 PLL loop filter was spread across three corners of the
package, which forced the charge-pump node onto an inner layer under the
GPS front end.  All three parts now sit in the pocket at U21's north-west
corner and the net stays on F.Cu.  A permanent RF_NO_DIGITAL area plus a
scoped rule keeps that region free of non-power copper on L3/L5/L6/B.Cu,
and the local router enforces the same restriction, since it is the one
stage that runs after the temporary keepout is deleted.

Also enables the solder-mask bridge check that was switched off
board-wide; the board passes it with no violations.

Final board: DRC clean - no shorts, clearance, disallowed-item or
dangling-copper violations - with 26 unconnected endpoints left.  Routing
is spread 37% L3, 27% F.Cu, 26% L5, 10% B.Cu over 6548 mm and 971 vias.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

outputs/drc_final_routed.rpt

@@ -0,0 +1,122 @@
+** Drc report for board_routed.kicad_pcb **
+** Created on 2026-08-16T01:33:28 **
+** Report includes: Errors, Warnings **
+
+** Found 0 DRC violations **
+
+** Found 26 unconnected pads **
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(106.4000 mm, 65.6000 mm): Pad D10 [MAX_I0] of U1 on F.Cu
+    @(154.6000 mm, 49.6900 mm): Pad 2 [MAX_I0] of R46 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(28.1950 mm, 51.2000 mm): Track [GND] on F.Cu, length 5.8400 mm
+    @(30.8500 mm, 51.9250 mm): Pad 2 [GND] of U6 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(175.0000 mm, 46.2500 mm): Pad 2 [GND] of J2 on B.Cu
+    @(175.0000 mm, 46.2500 mm): Pad 2 [GND] of J2 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(106.8000 mm, 64.4000 mm): Track [VCC3V0_DIG] on L3_SIG, length 1.4764 mm
+    @(112.0000 mm, 68.8000 mm): Pad H17 [VCC3V0_DIG] of U1 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(112.0000 mm, 68.8000 mm): Pad H17 [VCC3V0_DIG] of U1 on F.Cu
+    @(137.0000 mm, 70.0000 mm): Track [VCC3V0_DIG] on F.Cu, length 0.9050 mm
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(153.0459 mm, 42.6000 mm): Track [VCC3V0_RF] on F.Cu, length 2.3988 mm
+    @(153.1000 mm, 44.0000 mm): Pad 2 [VCC3V0_RF] of U21 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(150.4900 mm, 44.5000 mm): Pad 1 [MAX_CLKOUT_SRC] of R45 on F.Cu
+    @(153.1000 mm, 45.0000 mm): Pad 4 [MAX_CLKOUT_SRC] of U21 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(122.0100 mm, 65.5000 mm): Pad 2 [MAX_SCLK_DEV] of R51 on F.Cu
+    @(154.0000 mm, 47.4000 mm): Pad 8 [MAX_SCLK_DEV] of U21 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(155.0000 mm, 47.4000 mm): Pad 10 [MAX_Q1_DEV] of U21 on F.Cu
+    @(158.2000 mm, 50.7100 mm): Pad 1 [MAX_Q1_DEV] of R49 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(37.5251 mm, 104.2026 mm): Track [EXT5_PROT] on F.Cu, length 1.3496 mm
+    @(42.2500 mm, 104.0000 mm): Pad 6 [EXT5_PROT] of U8 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(44.2500 mm, 59.0000 mm): Pad 24 [FTDI_B_TDO_SRC] of U2 on F.Cu
+    @(96.5100 mm, 70.0000 mm): Pad 2 [FTDI_B_TDO_SRC] of R73 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(47.0000 mm, 56.7500 mm): Pad 32 [FTDI_C_TXD_SRC] of U2 on F.Cu
+    @(62.8625 mm, 54.8250 mm): Pad 15 [FTDI_C_TXD_SRC] of U29 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(47.0000 mm, 51.7500 mm): Pad 42 [FTDI_D_TXD_SRC] of U2 on F.Cu
+    @(62.8625 mm, 53.5250 mm): Pad 17 [FTDI_D_TXD_SRC] of U29 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(105.6000 mm, 65.6000 mm): Track [VCC3V3] on F.Cu, length 0.5657 mm
+    @(108.8000 mm, 64.0000 mm): Track [VCC3V3] on F.Cu, length 0.5657 mm
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(110.4000 mm, 67.2000 mm): Pad F15 [VCC3V3] of U1 on F.Cu
+    @(112.0000 mm, 75.2000 mm): Pad T17 [VCC3V3] of U1 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(110.4000 mm, 67.2000 mm): Track [VCC3V3] on F.Cu, length 0.5657 mm
+    @(108.8000 mm, 64.0000 mm): Track [VCC3V3] on F.Cu, length 0.5657 mm
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(112.8000 mm, 76.8000 mm): Track [VCC3V3] on F.Cu, length 0.5657 mm
+    @(112.0000 mm, 75.2000 mm): Track [VCC3V3] on F.Cu, length 0.5657 mm
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(113.2000 mm, 77.2000 mm): Via [VCC3V3] on F.Cu - B.Cu
+    @(119.3969 mm, 80.9333 mm): Track [VCC3V3] on F.Cu, length 0.3913 mm
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(111.2000 mm, 67.2000 mm): Pad F16 [MAX_SCLK] of U1 on F.Cu
+    @(120.9900 mm, 65.5000 mm): Pad 1 [MAX_SCLK] of R51 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(32.5100 mm, 58.0000 mm): Pad 2 [FTDI_OSCI] of R67 on F.Cu
+    @(39.0000 mm, 52.7500 mm): Pad 3 [FTDI_OSCI] of U2 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(39.0000 mm, 55.7500 mm): Pad 9 [FTDI_VPLL] of U2 on F.Cu
+    @(47.7170 mm, 58.2000 mm): Track [FTDI_VPLL] on L5_SIG, length 10.0820 mm
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(106.4000 mm, 68.0000 mm): Pad G10 [HARD_TCK] of U1 on F.Cu
+    @(96.9700 mm, 65.7400 mm): Track [HARD_TCK] on F.Cu, length 0.6505 mm
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(107.2000 mm, 67.2000 mm): Pad F11 [MAX_LD] of U1 on F.Cu
+    @(160.5000 mm, 46.9900 mm): Pad 2 [MAX_LD] of R50 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(83.0800 mm, 24.0000 mm): PTH pad 3 [PMOD1_3] of JPMOD1
+    @(108.8000 mm, 63.2000 mm): Pad A13 [PMOD1_3] of U1 on F.Cu
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(112.0000 mm, 63.2000 mm): Pad A17 [PMOD3_8] of U1 on F.Cu
+    @(116.5400 mm, 26.5400 mm): PTH pad 8 [PMOD3_8] of JPMOD3
+[unconnected_items]: Missing connection between items
+    Local override; error
+    @(111.2000 mm, 68.0000 mm): Pad G16 [MAX_SDATA] of U1 on F.Cu
+    @(120.9900 mm, 67.9000 mm): Pad 1 [MAX_SDATA] of R53 on F.Cu
+
+** Found 0 Footprint errors **
+
+** Ignored checks **
+    - Footprint has no courtyard defined
+    - Track endpoint not centered on via
+    - Tuning profile track geometries
+    - Footprint doesn't match symbol's footprint filters
+    - Footprint component type doesn't match footprint pads
+
+** End of Report **