Implement GameTime and the 12-month calendar

The Hollowdark calendar is 12 × 30-day months + a 5-day year-end
festival = 365 days, with 7-day weeks (docs/01-world.md and the
style bible). Months live in canonical order:

  spring    1  Thawing       2  Greening       3  Blossomtide
  summer    4  Highsun       5  Amberhaze      6  Harvestmark
  autumn    7  Firstfall     8  Stormturn      9  Ashfall
  winter   10  Rainfall     11  Hollowdark    12  Rimefrost
  festival 13  Year's End Festival (5 days)

  time/calendar.ts     month names, season mapping, days-per-month,
                       festival helpers
  time/gameTime.ts     GameTime shape + arithmetic: makeGameTime,
                       addDays / addWeeks / addMonths / addYears,
                       daysBetween / weeksBetween, compare / isBefore /
                       isAfter / isSameDay, dayOfWeek, dayOfYear,
                       toAbsoluteDays, formatGameTime
  time/granularity.ts  LifeStage + TickUnit mapping — one tick is a
                       year in infancy, a season in childhood, a month
                       in adolescence and old age, a week in adult life
                       (docs/05-time-system.md, ARCHITECTURE.md §5)
  time/speed.ts        Speed type: 'paused' | 'play' | 'fast'
  time/index.ts        public re-exports

Arithmetic is implemented by flattening GameTime to absolute-day
integers (year × 365 + dayOfYear - 1) so addDays, daysBetween, and
ordering are exact integer math. addMonths uses 13-month modular
arithmetic and clamps the day into the festival's 5-day length on
overflow.

59 unit tests in tests/unit/time/ cover constants, validation,
arithmetic edge cases (Rimefrost 30 → Festival 1, Festival 5 →
Thawing 1 of next year, day-clamping when landing in the festival),
negative offsets, tickOfDay preservation, day-of-week stability,
and life-stage boundaries.

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