docs(openfnirs): 5/8 sensor modules assembled — reflect onto ECU sensor-group headers

[zubyul]

What

Records the 2026-08-20 assembly milestone in the canonical OpenNIRScap build tree (bci/devices/openfnirs-build.tree):

• Sensor modules 1–5 assembled: J1 (Samtec T1M-06-F-SH-L-K) hand-soldered, consuming all five in-hand connectors.
• Blocker B1 updated: boards 6–8 still open — procure 3× T1M-06-F-SH-L-K.
• ECU reflection: the 5/8 sensor state lands on the ECU as five T1M-10 sensor-group headers due for population (from the qty-8 stock) per the channel/cable map, with orientation + mux channel order to be recorded; the generated unfilled-location count for sensor-group headers drops 8→3 once placement evidence lands.
• New §11 progress subtree; §5 location-map and substitution rows updated in place — additions land on the canonical surface, no sibling files.

Per the governing state machine, N2 (sensor J1) is 5/8 complete; N2→N3 proceeds per-board for boards 1–5.

Finish-line

1. Buy 3× T1M-06-F-SH-L-K → assemble boards 6–8.
2. Populate the five matching ECU T1M-10 headers; record orientation and mux channel order.
3. WI-2 bench verification + WI-9 photo evidence for boards 1–5.

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