D25 — Decoupling capacitor affinity pseudo-nets

Decision

Decoupling capacitors are tied to the nearest IC supply pin (in the input placement) by a two-pin pseudo-net at signal weight (PNet::affinity), counted in the objective only; on by default, --no-decap-affinity turns it off

Alternatives

Hungarian assignment + distance constraint (doc 04 §2)

Reason

Power nets carry weight 1, so full placement moved decaps 4 → 16–19 mm from their ICs (place_intent.py). The pseudo-net keeps every placer stage (B2B, annealer, LNS, bound) exact and unchanged and brings decaps to 1.8–3 mm (closer than the human layouts) for 1–9 % more HPWL. Assignment by optimisation is still open.