chore(tb): remove Verilator TBs + framework; parallelize XSIM runs

Verilator is no longer used (all verification is via Vivado XSIM). Remove:
- 10 per-module tb_*.cpp Verilator testbenches
- the entire test_framework/ Verilator harness (lib/, run_all.py, config.json,
  per-module test_plan.json/gen_vectors.py, golden vectors, reports)
- stale specs: verilator-conventions.md, test_framework/structure.md
  (index.md updated to drop the Verilator entry)

Parallelize run_tb.sh K x case execution (modules stay serial):
- new run_xsim_jobs helper: compile+elaborate once (serial, populates the
  shared xsim.dir), then run each (K,case) xsim in its own private workdir
  with a COPY of xsim.dir (~1MB) so concurrent same-snapshot runs don't clobber
  each other's runtime logs. Each workdir symlinks the repo sync_rtl tree so
  the TB's repo-relative $readmemh vector paths resolve.
- top/enc/dec runners refactored to build a (snapshot:K:case) spec list and
  hand it to run_xsim_jobs; ordered PASS/FAIL summary + per-job /tmp logs
  preserved. Bare './run_tb.sh top' now also takes the parallel path.

Speedup (20 cores): top full sweep 2:11 -> 0:51 (~2.6x), ~320% CPU.
Verified: top (11) / enc (9) / dec (9) all PASS; missing-vector runs still
fail (file-not-found guard -> exit 1).

sync_rtl/storage/TB/tb_storage.cpp

@@ -1,111 +0,0 @@
-// tb_storage.cpp - Verilator C++ testbench for storage (sd_bram top)
-//
-// Reads test vectors from +VECTOR_FILE= hex file.
-// Format: "ADDR DATA" per line (addr in 2-char hex, data in 12-char hex).
-// Writes values via sd_bram, reads back, prints "RESULT: VAL_HEX".
-// s_bram is compiled as rtl_dep.
-//
-// Clock: 10ns period. W=48, D=64, A=6 (default sd_bram parameters).
-
-#include <iostream>
-#include <fstream>
-#include <string>
-#include <sstream>
-#include <vector>
-#include <cstdint>
-#include "Vsd_bram.h"
-#include "verilated.h"
-
-#define CLK_HALF_PS 5000
-
-static vluint64_t main_time = 0;
-
-double sc_time_stamp() {
-    return (double)main_time / 1000.0;
-}
-
-static void tick(Vsd_bram* dut) {
-    dut->clk = 1;
-    main_time += CLK_HALF_PS;
-    dut->eval();
-    dut->clk = 0;
-    main_time += CLK_HALF_PS;
-    dut->eval();
-}
-
-int main(int argc, char** argv) {
-    Verilated::commandArgs(argc, argv);
-
-    const char* vector_file = NULL;
-    for (int i = 1; i < argc; i++) {
-        std::string arg(argv[i]);
-        if (arg.rfind("+VECTOR_FILE=", 0) == 0) {
-            vector_file = argv[i] + 13;
-            break;
-        }
-    }
-
-    if (!vector_file) {
-        std::cerr << "ERROR: +VECTOR_FILE= not specified" << std::endl;
-        return 1;
-    }
-
-    std::ifstream infile(vector_file);
-    if (!infile.is_open()) {
-        std::cerr << "ERROR: Cannot open vector file: " << vector_file << std::endl;
-        return 1;
-    }
-
-    struct Vec { int addr; uint64_t data; };
-    std::vector<Vec> vectors;
-
-    std::string line;
-    while (std::getline(infile, line)) {
-        if (line.empty() || line[0] == '#') continue;
-        std::istringstream iss(line);
-        int a;
-        uint64_t d;
-        if (!(iss >> std::hex >> a >> d)) continue;
-        if (a < 0 || a > 63) continue;
-        vectors.push_back({a, d & 0xFFFFFFFFFFFFULL});
-    }
-    infile.close();
-
-    if (vectors.empty()) {
-        std::cerr << "ERROR: No valid vectors in file" << std::endl;
-        return 1;
-    }
-
-    Vsd_bram* dut = new Vsd_bram;
-
-    dut->clk     = 0;
-    dut->rd_addr = 0;
-    dut->wr_en   = 0;
-    dut->wr_addr = 0;
-    dut->wr_data = 0;
-    dut->eval();
-    main_time += CLK_HALF_PS;
-    dut->eval();
-
-    // --- WRITE PHASE ---
-    for (size_t i = 0; i < vectors.size(); i++) {
-        dut->wr_en   = 1;
-        dut->wr_addr = vectors[i].addr & 0x3F;
-        dut->wr_data = vectors[i].data & 0xFFFFFFFFFFFFULL;
-        tick(dut);
-    }
-    dut->wr_en   = 0;
-    dut->wr_addr = 0;
-    dut->wr_data = 0;
-    tick(dut);
-
-    // --- READ PHASE ---
-    for (size_t i = 0; i < vectors.size(); i++) {
-        dut->rd_addr = vectors[i].addr & 0x3F;
-        tick(dut);
-        printf("RESULT: %012lX\n", (unsigned long)(dut->rd_data & 0xFFFFFFFFFFFFULL));
-    }
-
-    delete dut;
-    return 0;
-}