phase4: service-mode latency bench + mechanical ROI report generator

- bench-runner service mode (SPEC §3.1 mode c): a coordinated-omission-
  correct, open-loop, fixed-rate load generator; latency measured from
  the intended send time so queueing is counted, not hidden. Interleaved
  A/B of two server binaries, exact percentiles (bounded sample, no HDR
  bucketing error), bootstrap p50/p99 CIs via the existing ratio-CI
  machine. service-calibrate: A/A false-positive + injected latency-
  regression detection (calibration in progress).
- targets/cjson/service.c: minimal single-threaded HTTP server wrapping
  the patched cJSON.c (trust-layer, outside the workspace allowlist);
  env-toggled per-request busy-wait for the injection calibration only.
- report generator (crates/report bin): reads a ledger row +
  optional service-latency JSON, renders throughput→cores→dollars and/or
  latency percentiles, every figure with its CI, pricing source, and the
  methodology printed inline (SPEC §9); flags any non-accepted row so a
  rejected change never reads as a committed saving.
- ledger::report_row; justfile service/service-calibrate/report.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EcKPWQ3MQD4NdEHwar96u2

Cargo.lock

@@ -77,6 +77,7 @@ dependencies = [
  "anyhow",
  "clap",
  "ledger",
+ "libc",
  "serde",
  "serde_json",
  "toml",
@@ -373,6 +374,7 @@ name = "report"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "clap",
  "ledger",
  "serde",
  "serde_json",