Add the missing TPS2121 status pull-up (R108) and correct the power-mux docs

The power mux's status output had no pull-up, which broke the USB current
budget interlock.

TPS2121 ST is an open-drain output; the datasheet lists RST = 6-20 kOhm under
Recommended Operating Conditions, alongside RILM, so it is an external part.
ST is driven low only while IN2 (the J3 external 5 V header) is sourcing OUT,
and is high impedance whenever IN1 (the J7 USB-C input) is sourcing. U10 gate 2
inverts MUX_ST into EXT_SELECTED, and MAIN_EN = USB_3A_OK OR EXT_SELECTED
enables the 15 A core buck. With MUX_ST floating on USB-C power, a drift low
asserts MAIN_EN no matter what the TUSB320 advertised, so the board would try
to bring up the core rail on a 500 mA port. Every other open-drain net here
already has its pull-up (R7/R8, R30, R33/R34, R98/R104/R105/R106); this one was
missed.

R108 (10 k, VCC3V3_AUX -> MUX_ST) is appended at the end of design_data.py
rather than beside the mux: reference designators are assigned in call order,
so inserting it mid-file would renumber every later resistor and invalidate the
routed board. Placed at (53.0, 101.6), 2.4 mm from the U10 gate it holds up,
on a site checked clear of existing copper, and routed with close_remaining.py.

Also corrects POWER_TREE.md, which credited PR1 for giving the external input
priority. PR1 = GND actually *disables* priority mode; CP2 (tied to VCC3V3_AUX)
is what selects IN2. Since CP2 derives from the mux's own output the behaviour
is two-phase - highest-voltage wins at cold start, IN2 unconditionally once the
aux rail is up - so with both inputs connected there can be one seamless
switchover at startup. Documented, with the numbers: both eFuses UV 3.98 V /
OV 5.80 V, ILIM 2.69 A (USB-C) and 4.04 A (header), mux OV 5.69 V / ILIM 4.5 A.
The two inputs are never summed: both eFuses block reverse current and the mux
bonds exactly one input at a time.

board_routed.kicad_sch/.kicad_sym are now gitignored byte copies of the tracked
schematic; kicad-cli needs them beside the board for --schematic-parity, and
deleting them in the previous commit had silently disabled that check.

Verified: ERC 0/0, DRC 0 violations, 0 unconnected, every net one copper group,
schematic parity unchanged at the 402 pre-existing warnings.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012KaTmCCxCEW4awsmXRNT1d

outputs/bom.csv

@@ -101,6 +101,7 @@ R103,1,1K,Yageo,RC0402FR-071KL,Resistor_SMD:R_0402_1005Metric,SMT,no,External-re
 R104,1,10K,Yageo,RC0402FR-0710KL,Resistor_SMD:R_0402_1005Metric,SMT,no,LM3880 final-flag pull-up for all-power logic
 R105,1,10K,Yageo,RC0402FR-0710KL,Resistor_SMD:R_0402_1005Metric,SMT,no,1.8-V power-good pull-up for all-power logic
 R106,1,10K,Yageo,RC0402FR-0710KL,Resistor_SMD:R_0402_1005Metric,SMT,no,3.3-V power-good pull-up for all-power logic
+R108,1,10K,Yageo,RC0402FR-0710KL,Resistor_SMD:R_0402_1005Metric,SMT,no,"TPS2121 open-drain status pull-up (RST, 6-20 kohm)"
 "R11,R16",2,232K,Yageo,RC0402FR-07232KL,Resistor_SMD:R_0402_1005Metric,SMT,no,4.0-V nominal eFuse UV divider top
 "R12,R17",2,100K,Yageo,RC0402FR-07100KL,Resistor_SMD:R_0402_1005Metric,SMT,no,eFuse UV divider bottom
 "R13,R18",2,383K,Yageo,RC0402FR-07383KL,Resistor_SMD:R_0402_1005Metric,SMT,no,5.8-V nominal eFuse OV divider top
@@ -209,4 +210,4 @@ U35,1,TUSB320LAI,Texas Instruments,TUSB320LAIRSW(R),Package_DFN_QFN:Texas_X2QFN-
 U5,1,TUSB320LAI,Texas Instruments,TUSB320LAIRSW(R),Package_DFN_QFN:Texas_X2QFN-12_1.6x1.6mm_P0.4mm,SMT,no,USB Type-C port controller in UFP GPIO mode; integrated Rd on CC1/CC2; current-advertisement outputs unused since this port never sources SYS5
 U6,1,TPD2EUSB30A,Texas Instruments,TPD2EUSB30ADRTR,Package_TO_SOT_SMD:Texas_DRT-3,SMT,no,Low-capacitance USB 2.0 ESD protection
 "U7,U8",2,TPS259470A,Texas Instruments,TPS259470ARPWR,board:Texas_RPW0010A_QFN10,SMT,no,True-reverse-blocking eFuse; UV/OV/current/inrush protection
-U9,1,TPS2121,Texas Instruments,TPS2121RUXR,board:Texas_RUX0012A_VQFN12,SMT,no,4.5-A power mux; external input wins near-equal valid 5-V inputs
+U9,1,TPS2121,Texas Instruments,TPS2121RUXR,board:Texas_RUX0012A_VQFN12,SMT,no,4.5-A power mux.  PR1=GND disables priority mode; CP2 tied to VCC3V3_AUX is what selects IN2 (external 5 V) once the aux rail is up