fix(audit): enforce phase0 summary max-limit semantics

Validate maxSessions/maxChannels/maxSkipReasons as non-negative finite values, make 0 produce zero rows, and add regression coverage. No architecture/protocol flow changes; diagram files reviewed and no updates were needed.

docs/plans/state.json

@@ -463,6 +463,18 @@
       ],
       "test_status": "pnpm test:run src/audit/phase0BaselineDrift.test.ts + pnpm typecheck passing"
     },
+    "phase0-live-baseline-summary-limit-validation-hardening": {
+      "status": "completed",
+      "date": "2026-02-27",
+      "updated": "2026-02-27",
+      "summary": "Hardened phase-0 baseline summary limit handling so `maxSessions`, `maxChannels`, and `maxSkipReasons` enforce non-negative finite values, treat `0` as zero rows, and reject negative limits with explicit errors.",
+      "files_modified": [
+        "src/audit/phase0BaselineSummary.ts",
+        "src/audit/phase0BaselineSummary.test.ts",
+        "docs/plans/state.json"
+      ],
+      "test_status": "pnpm test:run src/audit/phase0BaselineSummary.test.ts + pnpm typecheck passing"
+    },
     "phase0-instrumentation-ticket-checklist": {
       "status": "completed",
       "date": "2026-02-25",

src/audit/phase0BaselineSummary.test.ts

@@ -163,6 +163,65 @@ describe('summarizePhase0Baseline', () => {
     expect(summary.run_outcomes.by_channel).toHaveLength(1);
     expect(summary.run_outcomes.by_channel[0]?.key).toBe('telegram');
   });
+
+  it('applies max row limits including zero', () => {
+    const events: AuditEvent[] = [
+      makeEvent(1000, 'run.state', {
+        session_id: 's1',
+        channel: 'telegram',
+        sender: 'u1',
+        source: 'channel',
+        state: 'complete',
+      }),
+      makeEvent(1100, 'run.state', {
+        session_id: 's2',
+        channel: 'discord',
+        sender: 'u2',
+        source: 'gateway',
+        state: 'error',
+      }),
+      makeEvent(1200, 'reaction.skip', {
+        session_id: 's1',
+        channel: 'telegram',
+        sender: 'u1',
+        source: 'channel',
+        reason: 'no_match',
+      }),
+      makeEvent(1300, 'reaction.skip', {
+        session_id: 's2',
+        channel: 'discord',
+        sender: 'u2',
+        source: 'gateway',
+        reason: 'no_rules',
+      }),
+    ];
+
+    const none = summarizePhase0Baseline(events, {
+      maxChannels: 0,
+      maxSessions: 0,
+      maxSkipReasons: 0,
+    });
+    expect(none.run_outcomes.by_channel).toHaveLength(0);
+    expect(none.run_outcomes.by_session).toHaveLength(0);
+    expect(none.reactions.skip_reasons).toHaveLength(0);
+
+    const oneEach = summarizePhase0Baseline(events, {
+      maxChannels: 1,
+      maxSessions: 1,
+      maxSkipReasons: 1,
+    });
+    expect(oneEach.run_outcomes.by_channel).toHaveLength(1);
+    expect(oneEach.run_outcomes.by_session).toHaveLength(1);
+    expect(oneEach.reactions.skip_reasons).toHaveLength(1);
+  });
+
+  it('rejects negative max limits', () => {
+    const events: AuditEvent[] = [];
+
+    expect(() => summarizePhase0Baseline(events, { maxSessions: -1 })).toThrow('maxSessions');
+    expect(() => summarizePhase0Baseline(events, { maxChannels: -1 })).toThrow('maxChannels');
+    expect(() => summarizePhase0Baseline(events, { maxSkipReasons: -1 })).toThrow('maxSkipReasons');
+  });
 });
 
 describe('renderPhase0BaselineMarkdown', () => {

src/audit/phase0BaselineSummary.ts

@@ -119,6 +119,19 @@ function toPct(part: number, whole: number): number | null {
   return Math.round((part / whole) * 10000) / 100;
 }
 
+function normalizeLimit(value: number | undefined, optionName: string): number | undefined {
+  if (value === undefined) {
+    return undefined;
+  }
+  if (!Number.isFinite(value)) {
+    throw new Error(`${optionName} must be a finite number.`);
+  }
+  if (value < 0) {
+    throw new Error(`${optionName} must be greater than or equal to 0.`);
+  }
+  return Math.floor(value);
+}
+
 function percentile(sortedAscending: number[], pct: number): number {
   if (sortedAscending.length === 0) {
     return 0;
@@ -183,7 +196,7 @@ function sortGroups(groups: Map<string, RunCounter>, limit?: number): RunOutcome
       return a.key.localeCompare(b.key);
     });
 
-  if (typeof limit === 'number' && Number.isFinite(limit) && limit > 0) {
+  if (typeof limit === 'number' && Number.isFinite(limit)) {
     return rows.slice(0, Math.floor(limit));
   }
   return rows;
@@ -199,7 +212,7 @@ function sortSkipReasons(reasons: Map<string, number>, limit?: number): Reaction
       pct: total > 0 ? Math.round((count / total) * 10000) / 100 : 0,
     }));
 
-  if (typeof limit === 'number' && Number.isFinite(limit) && limit > 0) {
+  if (typeof limit === 'number' && Number.isFinite(limit)) {
     return rows.slice(0, Math.floor(limit));
   }
   return rows;
@@ -244,6 +257,10 @@ export function summarizePhase0Baseline(
   events: AuditEvent[],
   options: Phase0BaselineSummaryOptions = {},
 ): Phase0BaselineSummary {
+  const maxSessions = normalizeLimit(options.maxSessions, 'maxSessions');
+  const maxChannels = normalizeLimit(options.maxChannels, 'maxChannels');
+  const maxSkipReasons = normalizeLimit(options.maxSkipReasons, 'maxSkipReasons');
+
   const sessionFilter = new Set(options.sessionIds ?? []);
   const channelFilter = new Set(options.channels ?? []);
   const senderFilter = new Set(options.senders ?? []);
@@ -342,8 +359,8 @@ export function summarizePhase0Baseline(
     event_counts: eventCounts,
     run_outcomes: {
       overall: buildRunOutcomeStats(runTotals),
-      by_channel: sortGroups(runByChannel, options.maxChannels),
+      by_channel: sortGroups(runByChannel, maxChannels),
-      by_session: sortGroups(runBySession, options.maxSessions),
+      by_session: sortGroups(runBySession, maxSessions),
     },
     cancel_latency_ms: computeLatencyStats(cancelLatencies),
     reactions: {
@@ -352,7 +369,7 @@ export function summarizePhase0Baseline(
       total: totalReactions,
       match_rate_pct: toPct(reactionMatched, totalReactions),
       skip_rate_pct: toPct(reactionSkipped, totalReactions),
-      skip_reasons: sortSkipReasons(skipReasons, options.maxSkipReasons),
+      skip_reasons: sortSkipReasons(skipReasons, maxSkipReasons),
     },
   };
 }