fix(ifind): legal A-share windows across a long closure are refused

[FlyM1ss]

Follow-up from #474 item 1 (PR #477). Known and documented in the code; filing so it is tracked rather than only commented.

minimum_bars_for_window derives its floor from weekdays, which cannot see exchange holidays. A window whose data is complete and correct is refused as truncated when more than half its weekdays fall inside a closure.

floor = max(4, weekdays * 4 * 0.5) → refusal whenever real_trading_days < weekdays / 2.

Measured (sweep over every legal span, both real closures):

window	span	floor	real trading days	real bars
2026-09-28 → 2026-10-09	11d	18	4	16
2026-09-25 → 2026-10-08	13d	18	4	16
2024-02-05 → 2024-02-17	12d	20	4	16

Both National Day (Oct 1–7) and Spring Festival reproduce it.

Two things worth knowing before working this:

• It is not reachable at the full 14-day span. Refusals cluster at 11–13 days; a 14-day window picks up enough weekdays on either side of the closure to clear its own floor. The comment at ifind_ashare.py:31-42 implies the widest windows are affected — they are not.
• The existing sweep cannot catch it. test_minimum_bars_never_exceeds_the_weekday_derived_bar_count varies weekdays only, so no closure ever enters it. Renamed in PR fix: preflight the pipeline backtest window #477 to stop it claiming coverage it does not have; the separate mutation-strength gap in that same test is test: the A-share floor sweep does not catch a restored flat constant #482.

It fails in the safe direction — a visible refusal, not a short curve charted as real — which is why it shipped. Lowering _MINIMUM_BAR_COMPLETENESS is not the fix: it trades a visible false refusal for silent acceptance of a genuinely truncated reply, which is the case the guard exists for. The fix is a CN trading calendar, which ifind_ashare.py deliberately does not carry (it would be a second, unversioned holiday table beside the exchange's own). An iFinD-served calendar would avoid that.

User-visible today: formatBacktestError names both holidays as the likely cause, so the message is accurate — the run is still refused.