Fix contract resolver: prefer rolled contract on equal volume

Changes > to >= so when both contracts have 0 volume (e.g. off-hours),
the further-out standard month wins. Fixes SI resolving to SIJ (April,
non-standard) instead of SIK (May, standard delivery month).

Also fixes SI/SIL prefix collision by requiring month+digit after symbol.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/contract-resolver.ts

@@ -116,7 +116,12 @@ export async function resolveContracts(
                 { headers }
             );
             const contracts: ContractInfo[] = res.data ?? [];
-            const match = contracts.find((c) => c.name.startsWith(sym));
+            // Match symbol exactly: after the symbol prefix, next char must be a month code (A-Z) then a digit
+            const match = contracts.find((c) => {
+                if (!c.name.startsWith(sym)) return false;
+                const rest = c.name.slice(sym.length);
+                return /^[A-Z]\d/.test(rest);
+            });
             frontMonths[sym] = match ? {
                 id: match.id,
                 name: match.name,
@@ -193,8 +198,9 @@ export async function resolveContracts(
             const frontVol = volumes[front.name] ?? 0;
             const rolledVol = volumes[rolled.name] ?? 0;
 
-            if (rolledVol > frontVol) {
-                // Rolled contract has more volume — use it
+            if (rolledVol >= frontVol) {
+                // Rolled contract has equal or more volume — use it
+                // (equal includes both-zero case: prefer the further-out month)
                 results[sym] = {
                     ...rolled,
                     alternative: front.name,
@@ -203,7 +209,7 @@ export async function resolveContracts(
                 };
                 console.log(`[contract-resolver] ${sym}: ${front.name} (vol=${frontVol}) → ${rolled.name} (vol=${rolledVol}) ROLLED`);
             } else {
-                // Front month still has more volume — keep it
+                // Front month has more volume — keep it
                 results[sym] = {
                     ...front,
                     alternative: rolled.name,