FR-4.5.1: a narrow window may not overturn a wide measured reading

The stability gate treated every evidence flip as symmetric, and that
produced a false downgrade on the candidate it was written for
(kotkan/claude-plugin-inference-arbitrage#15). Re-measuring
anxious/agent-wip/release-policy-derivation over four real windows with
the same script gave 0 -> 2 -> 12 -> 15 invocations at 7/14/27/60 days:
monotonic accumulation of a step that runs about once every two days, not
a label flipping about. The 7-day window had not caught an unstable
candidate, it had failed to observe a real one.

So a thin/unmeasured reading now only overturns a `measured` one when its
window is at least 90% as wide (observed since->until span). Below that
the comparison takes a third verdict, `insufficient-window`: not filed,
not downgraded to a boundary question, no FR-6.5 self-report, and carried
forward with the wider snapshot left as the standing comparison point.
The parity is relative rather than an absolute day count because the
adequate width is a property of the candidate's invocation rate, which
the auditor does not know in advance.

Unchanged, and tested: the reverse direction (thin prior -> measured now)
still downgrades, a flip between comparably wide windows still downgrades,
and a threshold crossing with `measured` on both sides is never excused by
narrowness. An unreadable window is not an exemption either — the flip
stands and window_parity records that the check could not run.

- bin/stability-classify: window_days/window_parity/deferrable, the
  insufficient-window verdict, stability_deferrals, window_parity on
  findings too, insufficient_window + window_parity_ratio in the gate block
- bin/audit-snapshot: record deferrals in the snapshot notes so the row is
  visible on the wiki page instead of silently dropped
- design/spec.md: FR-4.5.1 as a testable requirement; S4 restated
- design/rubric.md: the third boundary in §5b
- skills/offload-audit/SKILL.md: do not file and do not self-report a deferral
- tests: real 60-day anxious fixture; cases (f)/(f2) on real data, (j)-(n)
  synthetic — 7 test files pass

Verified against the live store (~/.cache/inference-arbitrage/wiki): the
real 7.0d-vs-26.49d comparison now reports insufficient-window with zero
findings, while the real 33.4d-vs-26.49d one reports stable at 13.87% and
files normally.

.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
 	"name": "inference-arbitrage",
-	"version": "0.6.3",
+	"version": "0.7.0",
 	"description": "Audits a Claude Code plugin's definitions and usage transcripts to find steps that should be a deterministic script instead of raw LLM inference, and files the well-evidenced ones as issues on the target repo.",
 	"author": {
 		"name": "oleks"