fix(audit): require integer gateway window padding

Enforce non-negative integer padding_ms/--window-padding-ms across gateway window helper and live baseline capture CLI, with regression coverage for invalid values. Architecture/protocol diagrams reviewed; no updates were needed for this validation-only change.

docs/plans/state.json

@@ -520,6 +520,19 @@
       ],
       "test_status": "pnpm test:run src/audit/phase0BaselineDrift.test.ts + pnpm typecheck passing"
     },
+    "phase0-live-baseline-gateway-window-padding-integer-validation": {
+      "status": "completed",
+      "date": "2026-02-27",
+      "updated": "2026-02-27",
+      "summary": "Hardened gateway cancel-window auto-detection padding by requiring non-negative integer `padding_ms`/`--window-padding-ms` values in both helper and capture CLI, preventing silent flooring of fractional millisecond values.",
+      "files_modified": [
+        "src/audit/phase0GatewayWindow.ts",
+        "src/audit/phase0GatewayWindow.test.ts",
+        "scripts/capture-phase0-live-baseline.ts",
+        "docs/plans/state.json"
+      ],
+      "test_status": "pnpm test:run src/audit/phase0GatewayWindow.test.ts + pnpm typecheck passing"
+    },
     "phase0-instrumentation-ticket-checklist": {
       "status": "completed",
       "date": "2026-02-25",

scripts/capture-phase0-live-baseline.ts

@@ -39,7 +39,7 @@ function usage(): string {
     '  --backend <native|pi_embedded|...[,..]> Restrict sample to selected backends (via backend.route timeline)',
     '  --exclude-session-substring <text[,..]> Exclude sessions containing any substring (default: probe)',
     '  --auto-gateway-cancel-window            Auto-select latest gateway cancel/cancelled session window',
-    '  --window-padding-ms <number>            Milliseconds added before/after auto-selected window (default: 250)',
+    '  --window-padding-ms <integer>           Milliseconds added before/after auto-selected window (default: 250)',
     '  --raw-identifiers                       Keep raw session/sender/request IDs (default: anonymized)',
     '  --tag <YYYY-MM-DD>                      Output file tag (default: current date UTC)',
     '  --sample-out <path>                     Output JSONL sample path override',
@@ -202,10 +202,7 @@ async function main(): Promise<void> {
   const backendTargets = parseBackendTargets(values.backend);
   const excludeSessionSubstrings = parseCsv(values['exclude-session-substring']) ?? ['probe'];
   const autoGatewayCancelWindow = Boolean(values['auto-gateway-cancel-window']);
-  const windowPaddingMs = parseOptionalNumber(values['window-padding-ms'], '--window-padding-ms');
+  const windowPaddingMs = parseOptionalInteger(values['window-padding-ms'], '--window-padding-ms');
-  if (windowPaddingMs !== undefined && windowPaddingMs < 0) {
-    throw new Error('--window-padding-ms must be greater than or equal to 0.');
-  }
 
   if (autoGatewayCancelWindow && (values.since || values.until)) {
     throw new Error('--auto-gateway-cancel-window cannot be combined with --since/--until.');

src/audit/phase0GatewayWindow.test.ts

@@ -71,4 +71,14 @@ describe('findLatestGatewayCancelWindow', () => {
 
     expect(findLatestGatewayCancelWindow(events)).toBeNull();
   });
+
+  it('rejects invalid padding values', () => {
+    const events: AuditEvent[] = [
+      event(1000, 'run.cancel', { session_id: 's1', source: 'gateway' }),
+      event(1010, 'run.state', { session_id: 's1', source: 'gateway', state: 'cancelled' }),
+    ];
+
+    expect(() => findLatestGatewayCancelWindow(events, { padding_ms: -1 })).toThrow('padding_ms');
+    expect(() => findLatestGatewayCancelWindow(events, { padding_ms: 1.5 })).toThrow('padding_ms');
+  });
 });

src/audit/phase0GatewayWindow.ts

@@ -36,6 +36,22 @@ function isGatewayEvent(payload: Record<string, unknown>): boolean {
   return readString(payload.source) === 'gateway';
 }
 
+function normalizePaddingMs(value: number | undefined): number {
+  if (value === undefined) {
+    return 0;
+  }
+  if (!Number.isFinite(value)) {
+    throw new Error('padding_ms must be a finite integer greater than or equal to 0.');
+  }
+  if (!Number.isInteger(value)) {
+    throw new Error('padding_ms must be an integer greater than or equal to 0.');
+  }
+  if (value < 0) {
+    throw new Error('padding_ms must be greater than or equal to 0.');
+  }
+  return value;
+}
+
 export function findLatestGatewayCancelWindow(
   events: AuditEvent[],
   options: FindGatewayCancelWindowOptions = {},
@@ -104,10 +120,7 @@ export function findLatestGatewayCancelWindow(
     return null;
   }
 
-  const padRaw = options.padding_ms ?? 0;
+  const paddingMs = normalizePaddingMs(options.padding_ms);
-  const paddingMs = Number.isFinite(padRaw) && padRaw > 0
-    ? Math.floor(padRaw)
-    : 0;
 
   return {
     session_id: latest.session_id,