[v1] Collapse all keccak256 implementations onto a single canonical copy

[botcoinmoney]

Background

The Phase 1 follow-up cross-impl audit (PR #9) caught three real bugs in `packages/cortex/src/state/keccak256.ts` (RC pair order, RHO table, squeeze stepping). The same bugs were independently reproduced in four other places in the repo because each phase agent vendored its own pure-JS keccak:

Location	Phase	Fixed in
`packages/cortex/src/state/keccak256.ts`	Phase 1 (canonical)	PR #9
`benchmark/generators/keccak256_vendor.ts`	Phase 4	commit `1533d5c`
`test/e2e/phase-3/run.mjs` (inline)	Phase 3	commit `1533d5c`
`test/e2e/phase-4/run.mjs` (inline)	Phase 4	commit `1533d5c`
`test/e2e/phase-6/run.mjs` (inline)	Phase 6	commit `11f84ce`
`scripts/replay-reducer.mjs` (public replayer)	Phase 6	commit `11f84ce`

Three out of five vendored copies had the same three bugs. Only the Python second-impl, written from spec, caught it — exactly what §9 Phase 1 required two reference impls to do.

Risk

A consensus-critical primitive having five separate copies is a footgun. If a future change is applied to one and not the others, divergent state roots silently appear in V0+ and miners produce receipts that can't be verified on-chain.

Proposed fix (V1)

1. Single canonical implementation in `packages/cortex/src/state/keccak256.ts`.
2. Remove `benchmark/generators/keccak256_vendor.ts` and import from `@botcoin/cortex` (Phase 4 should use the workspace dep — currently isn't a workspace member; either add it or move the loaders into a workspace).
3. Replace inline keccak in every `test/e2e/phase-*/run.mjs` with an import. The runner already runs after `npm run build --workspaces --if-present`; `packages/cortex/dist/state/index.js` is always available.
4. `scripts/replay-reducer.mjs` likewise imports from `@botcoin/cortex` after a build.

Definition of done

• Single keccak implementation, imported everywhere.
• A grep test `grep -rE "function keccak256|const RC \[|const RHO " --include=".ts" --include=".mjs"` returns exactly one match (the canonical `keccak256.ts`).

Cross-references

• PR feat(py): Phase 1 follow-up — Python second reference impl + cross-impl parity #9 (Python second-impl + canonical fix)
• Commits `1533d5c` and `11f84ce` (vendored-copy fixes)
• ORGANISM_CORTEX_STATE_PLAN.md §9 Phase 1 — "two independent reference implementations compute identical roots"