feat(spor-c): add Q3 cache-prefix experiment harness + analyser [skip-docs]

Implements Spor C of post-v3.4.0 roadmap. Zero-dep harness measures
CLAUDE_CODE_FORK_SUBAGENT cache-prefix preservation across 3 fork-children
with identical --allowedTools at 150-250K parent context.

Harness uses --append-system-prompt-file (avoids stdin buffer cap at
>200K bytes) + --exclude-dynamic-system-prompt-sections (prevents
per-child cache-prefix divergence from cwd/env/git-status).

Companion analyser summarizes accumulated ultraexecute-stats.jsonl:
percentile wall_time (p50/p90/max), total events, ISO time range.
Output: JSON via --json <path> CLI shim.

Result file is gitignored (*.local.md). Master-plan thresholds
(<= 1.5K positive / >= 3.5K negative) gate the v3.5.0 Path C decision.

Brief: .claude/projects/2026-05-04-spor-c-q3-cache-prefix-experiment/brief.md
Master-plan: .claude/projects/2026-05-04-post-v3.4.0-roadmap/master-plan.md

plugins/ultraplan-local/lib/stats/cache-analyzer.mjs

@@ -0,0 +1,117 @@
+// lib/stats/cache-analyzer.mjs
+// Summarizes ultraexecute-stats.jsonl: total events, percentile wall times,
+// time range. Companion to event-emit.mjs (which produces the jsonl).
+//
+// Designed for /ultraplan-local Spor C: gives C3 telemetry context when
+// interpreting Q3 experiment numbers (5+ weeks of accumulated data on the
+// operator's machine as of 2026-05-04).
+//
+// Zero npm dependencies. Node stdlib only.
+
+import { readFileSync, existsSync } from 'node:fs';
+
+function usage() {
+  return `cache-analyzer.mjs — summarize ultraexecute-stats.jsonl
+
+USAGE:
+  node lib/stats/cache-analyzer.mjs --json <path-to-jsonl>
+
+OUTPUT (stdout, JSON):
+  {
+    "total_events": <n>,
+    "events_with_duration": <n>,
+    "wall_time_ms_p50": <ms or null>,
+    "wall_time_ms_p90": <ms or null>,
+    "wall_time_ms_max": <ms or null>,
+    "unique_event_names": [...],
+    "oldest_event_iso": "<iso8601 or null>",
+    "newest_event_iso": "<iso8601 or null>"
+  }
+
+EXIT:
+  0 success, 1 file not found / read error, 2 usage error.
+`;
+}
+
+export function summarize(lines) {
+  const summary = {
+    total_events: 0,
+    events_with_duration: 0,
+    wall_time_ms_p50: null,
+    wall_time_ms_p90: null,
+    wall_time_ms_max: null,
+    unique_event_names: [],
+    oldest_event_iso: null,
+    newest_event_iso: null,
+  };
+
+  const durations = [];
+  const names = new Set();
+  let oldestMs = null;
+  let newestMs = null;
+
+  for (const line of lines) {
+    const trimmed = line.trim();
+    if (trimmed === '') continue;
+    let obj;
+    try { obj = JSON.parse(trimmed); }
+    catch { continue; }
+    summary.total_events++;
+    if (obj.event && typeof obj.event === 'string') names.add(obj.event);
+    else if (obj.name && typeof obj.name === 'string') names.add(obj.name);
+    if (typeof obj.duration_ms === 'number' && Number.isFinite(obj.duration_ms)) {
+      durations.push(obj.duration_ms);
+      summary.events_with_duration++;
+    }
+    const tsField = obj.timestamp || obj.ts || obj.iso || obj.time;
+    if (typeof tsField === 'string') {
+      const t = Date.parse(tsField);
+      if (!Number.isNaN(t)) {
+        if (oldestMs === null || t < oldestMs) oldestMs = t;
+        if (newestMs === null || t > newestMs) newestMs = t;
+      }
+    }
+  }
+
+  if (durations.length > 0) {
+    durations.sort((a, b) => a - b);
+    const p50Idx = Math.floor(durations.length * 0.5);
+    const p90Idx = Math.floor(durations.length * 0.9);
+    summary.wall_time_ms_p50 = durations[Math.min(p50Idx, durations.length - 1)];
+    summary.wall_time_ms_p90 = durations[Math.min(p90Idx, durations.length - 1)];
+    summary.wall_time_ms_max = durations[durations.length - 1];
+  }
+
+  summary.unique_event_names = [...names].sort();
+  if (oldestMs !== null) summary.oldest_event_iso = new Date(oldestMs).toISOString();
+  if (newestMs !== null) summary.newest_event_iso = new Date(newestMs).toISOString();
+
+  return summary;
+}
+
+export function summarizeFile(path) {
+  if (!existsSync(path)) {
+    return { error: `file not found: ${path}` };
+  }
+  let text;
+  try { text = readFileSync(path, 'utf-8'); }
+  catch (e) { return { error: `read error: ${e.message}` }; }
+  return summarize(text.split('\n'));
+}
+
+if (import.meta.url === `file://${process.argv[1]}`) {
+  const args = process.argv.slice(2);
+  const jsonIdx = args.indexOf('--json');
+  if (jsonIdx === -1 || !args[jsonIdx + 1]) {
+    process.stderr.write(usage());
+    process.exit(2);
+  }
+  const path = args[jsonIdx + 1];
+  const result = summarizeFile(path);
+  if (result.error) {
+    process.stderr.write(`cache-analyzer: ${result.error}\n`);
+    process.exit(1);
+  }
+  process.stdout.write(JSON.stringify(result, null, 2) + '\n');
+  process.exit(0);
+}