feat(pyc): add semantic popcount primitive

[zhoubot]

Summary

• add vendor-neutral pyc.popcount with exact max(1, ceil(log2(N+1))) result width
• expose the same semantic operation through structural pyCircuit, Cycle-Aware Signal, and Agentic Circuit
• lower Agentic ACIR to one PYC operation and provide typed gfsim helper/SimQueue behavior
• select a digest-verified repository-owned BSD-3-Clause RTL candidate only in the Verilog backend
• use a padded balanced adder tree so implementation depth matches the logarithmic MLIR cost model
• remove the legacy Agentic hard-coded popcount Verilog path; no Solderpad source from PR feat(runtime): integrate Verilog crawler and catalog #29 is imported or relicensed

Decisions

• Decision 0161: qualified RTL selection remains backend-owned and fail closed
• Decision 0164: population count is a semantic PYC primitive

Validation

• pre-commit run
• pytest tests/unit -m unit — 20 passed
• pytest tests/system/test_primitive_selection.py -q — 11 passed
• Agentic Python focused lane — 142 passed, 2 skipped
• check-acir — 140/140
• ACIROpsTests — 1845/1845
• CodeGenTests — 97/97
• GfsimTests — 251/251
• repository contracts and API hygiene — passed
• mkdocs build --strict — passed
• decision-status validation — 164 rows, 0 deferred

Evidence: docs/gates/logs/20260905-pyc-popcount-rtl-r1/summary.md

Stack

This PR is intentionally based on #35 (codex/ac-stateful-rule) and should be reviewed/merged after the earlier stacked PRs.