perf: cancelled writes + ingestor I/O + threshold tests (#1120 follow-up) (#1167)

Red commit: e964ec9c46 (CI run: pending —
workflow only triggers on PR open)

Partial fix for #1120 — finishes the four follow-up items left open
after PR #1123 (cancelled writes, ingestor I/O, threshold-flag tests,
docs).

## What's done

- **`cancelledWriteBytesPerSec`** — server `/proc/self/io` parser
handles `cancelled_write_bytes`; `/api/perf/io` exposes the per-second
rate; Perf page renders it next to Read/Write with ⚠️ when sustained >1
MB/s.
- **Ingestor `/proc/<pid>/io`** — `cmd/ingestor/stats_file.go` samples
its own `/proc/self/io` each tick and includes `procIO` in the snapshot.
The server's `/api/perf/io` reads it and surfaces `.ingestor`. Frontend
renders an `Ingestor process` Disk I/O block alongside the existing
`server process` block (issue mockup: "Both ingestor and server").
- **Threshold + anomaly tests** — `test-perf-disk-io-1120.js` now
asserts ⚠️ fires/suppresses on WAL>100MB, cache_hit<90%, and the
backfill-rate-vs-tx-rate guard with the `tx_inserted >= 100` baseline
floor. Drops the tautological `|| ... === false` short-circuits flagged
in MINOR m4.
- **Docs (m8)** — `config.example.json` adds `_comment_ingestorStats`
(env var, default path, shared-tmp security note);
`cmd/ingestor/README.md` adds `CORESCOPE_INGESTOR_STATS` to the env-var
table plus a `Stats file` section.

## What's NOT done (deferred)

m1 sync.Map → map+RWMutex, m2 perfIOMu rate caching, m3 negative
cacheSize translation, m5 deterministic-write test, m7 ctx-aware
shutdown — pure polish; will file a follow-up issue if the operator
wants them tracked.

## TDD

- Red: `e964ec9` — adds failing tests + stub field/handler shape
(cancelled missing from struct, ingestor stub returns nil, ingestor
procIO absent).
- Green: `1240703` — wires up the parser case, ingestor sampler,
frontend rendering, docs.

E2E assertion added: test-perf-disk-io-1120.js:108

---------

Co-authored-by: clawbot <clawbot@users.noreply.github.com>
Co-authored-by: Kpa-clawbot <bot@kpa-clawbot.local>
Co-authored-by: Kpa-clawbot <bot@kpa-clawbot>

config.example.json

@@ -16,6 +16,7 @@
     "incrementalVacuumPages": 1024,
     "_comment": "vacuumOnStartup: run one-time full VACUUM to enable incremental auto-vacuum on existing DBs (blocks startup for minutes on large DBs; requires 2x DB file size in free disk space). incrementalVacuumPages: free pages returned to OS after each retention reaper cycle (default 1024). See #919."
   },
+  "_comment_ingestorStats": "Ingestor publishes a 1-Hz stats snapshot consumed by the server's /api/perf/io and /api/perf/write-sources endpoints (#1120). Path is configured via the CORESCOPE_INGESTOR_STATS environment variable on the INGESTOR process. Default: /tmp/corescope-ingestor-stats.json. The writer uses O_NOFOLLOW + 0o600, so a pre-planted symlink in /tmp cannot be used to clobber an arbitrary file. SECURITY: in shared-tmp environments (multi-tenant hosts), point CORESCOPE_INGESTOR_STATS at a private directory like /var/lib/corescope/ingestor-stats.json that only the corescope user can write to.",
   "https": {
     "cert": "/path/to/cert.pem",
     "key": "/path/to/key.pem",