core+gui+cli: anticipation/rebuild dates, preserve relative settings

Fixes around the delivery/build/check date handling (karen7 regtest):

- build_will (GUI + CLI) re-anchors date_to_check to the CURRENT heirs'
  earliest future delivery before building, so an anticipated rebuild is no
  longer blocked by the stale old-will anchor (NO_FUTURE_DATE). The checks of
  the existing will keep their anchored date_to_check.
- _sync_locktime_to_built_txs now PRESERVES RELATIVE locktime/threshold
  recipes ("2y"/"150d") in WILL_SETTINGS instead of freezing them to absolute
  timestamps: the anchored comparisons (resolve_locktime_against_tx and
  resolve_date_to_check with built_locktime) already prevent the daily
  invalidate prompt. Absolute values still sync on a genuine automatic
  anticipation.
- Will.remove_stale_wallet_history drops stale wallet-LOCAL will placeholders
  before every (re)build in GUI and CLI so their coins are available again.
- check_willexecutors_and_heirs raises HeirNotFoundException outside the
  count_heirs gate: a shortened relative recipe on a signed will now triggers
  a plain rebuild ("no heirs" only when there really are none).
- is_locktime_below_threshold compares the settings on one reference frame
  (resolve_guard_threshold), no longer against the built-will anchor.

Tests: purge unit + call-site, no-heirs, guard, anticipated-rebuild
end-to-end (GUI) + CLI mirror + fixed_percent_lists_amount unit,
relative-preserve sync; conftest restores electrum.constants.net after each
test (cross-file pollution guard).

tests/test_gui_prepare_will_history.py

@@ -179,6 +179,7 @@ def test_rebuild_path_schedules_full_refresh():
     win.date_to_check = 1_800_000_000
     win.will_settings = {"baltx_fees": 1, "locktime": "1 month"}
     win.bal_plugin = _CfgBag(
+        is_basic_mode=lambda: False,
         MAX_WILLEXECUTOR_FEE=_Cfg(1),
         SAVE_HISTORY=_Cfg(True),
         HISTORY_LABEL=_Cfg("LBL"),
@@ -204,6 +205,46 @@ def test_rebuild_path_schedules_full_refresh():
     schedule_mock.assert_called_once_with()
 
 
+def test_rebuild_purges_stale_wallet_history_before_building():
+    # The rebuild path must drop stale wallet-LOCAL will placeholders (saved by
+    # an earlier prepare) so their coins are available to the new build.
+    win = object.__new__(BalWindow)
+    win.disable_plugin = False
+    win.heirs = {"h": object()}
+    win.willexecutors = {}
+    win.no_willexecutor = True
+    win.willitems = {}
+    win.will = {}
+    win.date_to_check = 1_800_000_000
+    win.will_settings = {"baltx_fees": 1, "locktime": "1 month"}
+    win.bal_plugin = _CfgBag(
+        is_basic_mode=lambda: False,
+        MAX_WILLEXECUTOR_FEE=_Cfg(1),
+        SAVE_HISTORY=_Cfg(True),
+        HISTORY_LABEL=_Cfg("LBL"),
+    )
+    win.window = _FakeWindow()
+    win.window.wallet = _Wallet()
+    with (
+        patch.object(Util, "get_available_utxos", return_value=[]),
+        patch.object(Util, "parse_locktime_string", return_value=1_800_000_001),
+        patch.object(Will, "get_min_locktime", return_value=0),
+        patch.object(Will, "check_amounts"),
+        patch.object(BalWindow, "init_class_variables"),
+        patch.object(BalWindow, "build_will"),
+        patch.object(
+            BalWindow,
+            "check_will",
+            side_effect=[NotCompleteWillException(), None],
+        ),
+        patch.object(BalWindow, "update_all"),
+        patch.object(BalWindow, "_schedule_history_refresh"),
+        patch.object(Will, "remove_stale_wallet_history") as purge_mock,
+    ):
+        BalWindow.build_inheritance_transaction(win)
+    purge_mock.assert_called_once_with(win.window.wallet, "LBL")
+
+
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