feat(fusion_accounting_reports): pure-Python services for date+account+totaling

Three service modules with no Odoo dependencies:
- date_periods: fiscal year/month/quarter bounds + comparison derivation
- account_hierarchy: parent-child tree walker with type filtering
- totaling: move-line aggregation primitives

18 unit tests covering edge cases (December rollover, Feb 29, fiscal-
year-before-start, balance check tolerance).

Made-with: Cursor

fusion_accounting_reports/services/date_periods.py

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+"""Date period math for financial reports.
+
+Pure-Python helpers that compute:
+- Fiscal year start/end given any reference date + company fiscal year settings
+- Comparison periods (prior year same period, prior period, etc.)
+- Period boundaries for monthly / quarterly / yearly reporting
+
+NO Odoo imports - all callers pass in primitive types so the same module
+is unit-testable without an Odoo registry."""
+
+from dataclasses import dataclass
+from datetime import date, timedelta
+from typing import Literal
+
+
+PeriodGranularity = Literal['month', 'quarter', 'year', 'custom']
+ComparisonMode = Literal['none', 'previous_period', 'previous_year']
+
+
+@dataclass(frozen=True)
+class Period:
+    date_from: date
+    date_to: date
+    label: str
+
+    def __post_init__(self):
+        if self.date_from > self.date_to:
+            raise ValueError(f"date_from ({self.date_from}) > date_to ({self.date_to})")
+
+    @property
+    def days(self) -> int:
+        return (self.date_to - self.date_from).days + 1
+
+
+def fiscal_year_bounds(reference_date: date, *, fy_start_month: int = 1,
+                       fy_start_day: int = 1) -> Period:
+    """Return the fiscal year period containing `reference_date`.
+
+    Default: calendar year (Jan 1 - Dec 31). Pass fy_start_month=4, fy_start_day=1
+    for an April-March fiscal year."""
+    if reference_date.month < fy_start_month or (
+        reference_date.month == fy_start_month and reference_date.day < fy_start_day
+    ):
+        start_year = reference_date.year - 1
+    else:
+        start_year = reference_date.year
+    start = date(start_year, fy_start_month, fy_start_day)
+    next_start = date(start_year + 1, fy_start_month, fy_start_day)
+    end = next_start - timedelta(days=1)
+    return Period(date_from=start, date_to=end, label=f"FY {start_year}")
+
+
+def month_bounds(reference_date: date) -> Period:
+    """Return the calendar month containing `reference_date`."""
+    start = reference_date.replace(day=1)
+    if reference_date.month == 12:
+        next_start = date(reference_date.year + 1, 1, 1)
+    else:
+        next_start = date(reference_date.year, reference_date.month + 1, 1)
+    return Period(
+        date_from=start,
+        date_to=next_start - timedelta(days=1),
+        label=start.strftime('%B %Y'),
+    )
+
+
+def quarter_bounds(reference_date: date) -> Period:
+    """Return the calendar quarter containing `reference_date`."""
+    quarter = (reference_date.month - 1) // 3 + 1
+    start_month = (quarter - 1) * 3 + 1
+    start = date(reference_date.year, start_month, 1)
+    end_month = start_month + 2
+    if end_month == 12:
+        end = date(reference_date.year, 12, 31)
+    else:
+        end = date(reference_date.year, end_month + 1, 1) - timedelta(days=1)
+    return Period(date_from=start, date_to=end, label=f"Q{quarter} {reference_date.year}")
+
+
+def comparison_period(period: Period, mode: ComparisonMode) -> Period | None:
+    """Derive the comparison period for `period` per `mode`.
+
+    `previous_period`: same length, immediately before
+    `previous_year`: same calendar dates, one year earlier
+    `none`: returns None"""
+    if mode == 'none':
+        return None
+    if mode == 'previous_period':
+        days = period.days
+        new_to = period.date_from - timedelta(days=1)
+        new_from = new_to - timedelta(days=days - 1)
+        return Period(date_from=new_from, date_to=new_to,
+                      label=f"{period.label} (previous)")
+    if mode == 'previous_year':
+        try:
+            new_from = period.date_from.replace(year=period.date_from.year - 1)
+            new_to = period.date_to.replace(year=period.date_to.year - 1)
+        except ValueError:
+            new_from = period.date_from.replace(year=period.date_from.year - 1, day=28)
+            new_to = period.date_to.replace(year=period.date_to.year - 1, day=28)
+        return Period(date_from=new_from, date_to=new_to,
+                      label=f"{period.label} (prev year)")
+    raise ValueError(f"Unknown comparison mode: {mode}")