v3.6.1 — the dedup census line says the window is moving

A follow-up to 3.6.0, prompted by a real observation on a live brain.

3.6.0 made the dedup census window rotate instead of re-comparing a frozen prefix of the anchor
list. The mechanism worked — the cursor advanced and the run produced new alias links where every
earlier run produced zero. But the report still said:

[census truncated at anchor cap: 2000 of 3859 anchors compared]

which is exactly what the old, permanently-stuck census looked like. It states that work was cut
and nothing about the window advancing, so a healthy rotating census and a broken one rendered
identically — leaving no way to tell from the report that 3.6.0 had changed anything.

Fixed

• The dedup line now names the window, marks a wrap, and gives the coverage horizon:
  [census window 2000-3859 of 3859 anchors, wraps; rotates per run, full pass every 2 runs]
• dedup_window_start was written into the report and never rendered — the same
  recorded-but-invisible defect class 3.6.0 existed to remove. It had been placed on the
  counter-contract test's exemption list rather than surfaced, so the guard built to catch this
  was switched off for the one key that needed it. The exemption is gone and the contract test
  now covers it.

Scope

Report wording and a test guard. No behaviour change: cap, cursor, rotation and comparison logic
are untouched, and no data is migrated or deleted.