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