memory_libmap: Add beam search for many-port memories

The existing read port assignment algorithm uses Cartesian product
expansion, which has O(options^N) complexity. For memories with many
read ports (e.g., 64 parallel reads), this causes exponential memory
usage (60GB+) and timeouts.

This commit adds a beam search algorithm that activates for >8 read
ports. It maintains only the top K (default 16) configurations at
each step, reducing complexity to O(N * options * K).

Tested with:
- 64 read ports: completes in ~1s vs OOM
- 32 read ports: completes in ~1s vs timeout
- Maintains existing behavior for ≤8 ports

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/memlib/memlib_beam_search.v

@@ -0,0 +1,27 @@
+// Test case for beam search optimization in memory_libmap
+// This memory with 32 parallel read ports would cause exponential
+// blowup (O(4^32) = 10^19 configurations) without beam search pruning
+
+module memlib_beam_search (
+    input wire clk,
+    input wire we,
+    input wire [9:0] wr_addr,
+    input wire [7:0] wr_data,
+    input wire [9:0] base_addr,
+    output reg [255:0] parallel_out  // 32 x 8 = 256 bits
+);
+
+    reg [7:0] mem [0:1023];
+    integer i;
+
+    always @(posedge clk) begin
+        if (we)
+            mem[wr_addr] <= wr_data;
+
+        // 32 parallel reads - triggers beam search
+        for (i = 0; i < 32; i = i + 1) begin
+            parallel_out[i*8 +: 8] <= mem[base_addr + i];
+        end
+    end
+
+endmodule