docs(vacay): document the leave year and the comp/half modifiers

The page still described Vacay as strictly January to December and never
mentioned half days at all, so both new features would have shipped
undocumented.

wiki/Vacay.md

@@ -24,7 +24,21 @@ For each year, your entitlement panel shows:
 - **Used** — days you have logged on the calendar.
 - **Remaining** — entitlement plus any carried-over days, minus used days.
 
-**Carry-over** — when `carry_over_enabled` is on, unused days from the previous year are automatically added to the next year's total. When you toggle this setting, the carry-over amount is recalculated across all existing year records. Turn it off to zero out all carry-over balances.
+**Carry-over** — when `carry_over_enabled` is on, unused days from the previous period are automatically added to the next one's total. When you toggle this setting, the carry-over amount is recalculated across all existing year records. Turn it off to zero out all carry-over balances.
+
+## Leave year
+
+Not every leave year runs January to December, so you can set yours in Settings under **Vacation year**:
+
+- **Calendar** — 1 January to 31 December. The default, and what every existing plan uses.
+- **Fiscal** — starts on a month and day you choose, for example 1 July to 30 June.
+- **Hire date** — starts on the anniversary of the date you were hired.
+
+Entitlement, used days and carry-over are all counted over that period rather than the calendar year, and the calendar grid renders its twelve months accordingly — a July start shows Jul–Jun and rolls over the year boundary in the middle. The active period is spelled out under the entitlement heading so a card labelled "2026" is never ambiguous.
+
+This is a **personal** setting. In a fused plan each person keeps their own leave year and their own numbers; the grid follows whoever is looking at it, so everyone still sees the same days off.
+
+> If your leave year starts partway through a month (6 April, say), the twelve month cards are aligned to whole months while the day counting stays exact. The first days of the starting month are drawn but belong to the previous period.
 
 ## Calendar view
 
@@ -34,6 +48,15 @@ Days that overlap with any of your existing TREK trips are marked with a small b
 
 You can also switch the calendar toolbar to **Company** mode to mark shared company holidays, which are highlighted in amber and do not deduct from personal allowances.
 
+### Half days and comp days
+
+Two toggles in the toolbar change what a click logs. They are independent, so they combine:
+
+- **Half day** — logs the day as 0.5 instead of a full day. Half days keep the person's colour and carry a small orange dot in the corner.
+- **Comp / flex** — logs the day as time off in lieu (flextime, overtime taken back). It costs **nothing** from the vacation entitlement. Comp days render as a diagonal hatch of the person's colour instead of a solid fill, and are counted separately beside the entitlement tiles.
+
+Each toggle's icon in the toolbar is the marker it places, so you can see what a click will do before you make it. Clicking a day again with the same settings clears it; clicking with different ones converts it in place.
+
 **Settings** (gear icon) let you configure:
 
 - **Block weekends** — prevents logging on weekend days. You choose which days count as the weekend.