PROF=1: opt-in performance profile (latency percentiles, expert I/O, tuning verdict)

Answers 'where does it slow down on THIS machine with THIS config' so users
can tune RAM_GB/PIPE/DIRECT/PIN for their hardware without folklore:

- startup header: CPU/cores/RAM/backend + effective knobs (cache cap, pin,
  DRAFT/PIPE/DIRECT/MMAP/IDOT/DSA/PILOT/CACHE_ROUTE) — every saved log is
  self-describing when comparing runs across configs or machines
- per-forward decode latency ring (32k) -> p50/p90/p99/max, plus a tail
  diagnosis when p99 >> p50 (cold-cache expert loads)
- expert I/O accounting at the pread/mmap-touch sites: GB fetched, MB/token,
  GB/s, hit rate, loads/token, pinned/LRU tier fill
- phase shares of wall time and a plain-language verdict naming the knob
  most likely to move tok/s (I/O-bound vs compute-bound vs attention-bound)
- reports after REPLAY / PROMPT / oracle runs (stdout) and per turn in serve
  mode (stderr; stdout stays the framed protocol)

Additive only: with PROF unset every mode's output is byte-identical.
hwinfo_emit's /proc probe is factored into hw_probe() and shared.
Validated end-to-end on a tiny-random unquantized fixture (REPLAY, PROF on/
off, RAM_GB squeeze flips hit 96.9%->26.6% and the verdict follows);
make check clean, 0 warnings.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TCBNCciBaHea41QmidLUMn

docs/ENVIRONMENT.md

@@ -63,6 +63,7 @@ Format: `VAR` — default — effect.
 | `ABSORB` | `-1` (auto: absorbed for S≤4) | MLA attention absorption mode. |
 | `IDOT` | `1` | Integer dot-product kernel. `IDOT=0` uses exact f32 kernels (for A/B numerical checks). |
 | `COLI_POLICY` | `quality` | Resource policy: `quality`, `balanced`, or `experimental-fast`. |
+| `PROF` | `0` (off) | Performance profile: a startup header (machine + effective config), then per run — or per turn in serve mode, on stderr — forward-latency percentiles (p50/p90/p99/max), expert-I/O totals and cache-tier fill, phase shares of wall time, and a verdict naming the knob most likely to help on this machine. Output is additive; `PROF` unset changes nothing. |
 
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