docs: add wiki pages and wiki-sync workflow

.github/workflows/wiki-sync.yml

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+name: Sync Wiki
+
+on:
+  push:
+    branches: [main]
+    paths: [".wiki/**"]
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Checkout wiki
+        id: clone_wiki
+        run: |
+          if git clone "https://x-access-token:${{ github.token }}@github.com/${{ github.repository }}.wiki.git" .wiki-remote 2>&1; then
+            echo "wiki_exists=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "wiki_exists=false" >> "$GITHUB_OUTPUT"
+            echo "Wiki not yet initialized — skipping sync. Initialize via GitHub web UI first."
+          fi
+
+      - name: Sync wiki pages
+        if: steps.clone_wiki.outputs.wiki_exists == 'true'
+        run: |
+          cp .wiki/*.md .wiki-remote/
+          cd .wiki-remote
+          git config user.name "github-actions[bot]"
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git add -A
+          if git diff --cached --quiet; then
+            echo "No wiki changes to commit"
+          else
+            git commit -m "Sync wiki from repo"
+            git push origin HEAD:master
+          fi

.wiki/API-Reference.md

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+# API Reference
+
+## Conversion Functions
+
+### toHijri
+
+```dart
+HijriDate? toHijri(DateTime date, {ConversionOptions? options})
+```
+
+Converts a Gregorian date to a Hijri date. Returns `null` if the date falls outside the engine's supported range.
+
+The default engine is UAQ (Umm al-Qura), covering Hijri years 1318-1500 (Gregorian 1900-2076).
+
+```dart
+final hijri = toHijri(DateTime.utc(2025, 3, 1));
+// hijri.hy == 1446, hijri.hm == 9, hijri.hd == 1
+```
+
+### HijriDate fields
+
+| Field | Type | Description |
+| --- | --- | --- |
+| `hy` | `int` | Hijri year |
+| `hm` | `int` | Hijri month (1-12) |
+| `hd` | `int` | Hijri day (1-30) |
+
+---
+
+### toGregorian
+
+```dart
+DateTime? toGregorian(int hy, int hm, int hd, {ConversionOptions? options})
+```
+
+Converts a Hijri date to a Gregorian `DateTime` (UTC, midnight). Returns `null` if the date is outside the engine's supported range or is invalid.
+
+```dart
+final greg = toGregorian(1446, 9, 1);
+// greg.toIso8601String().substring(0, 10) == "2025-03-01"
+```
+
+---
+
+### isValidHijriDate
+
+```dart
+bool isValidHijriDate(int hy, int hm, int hd, {ConversionOptions? options})
+```
+
+Returns `true` if the given Hijri date is valid within the selected engine's calendar. Checks year range, month range (1-12), and day range (1 to the actual days in that month).
+
+---
+
+### daysInHijriMonth
+
+```dart
+int daysInHijriMonth(int hy, int hm, {ConversionOptions? options})
+```
+
+Returns the number of days in a Hijri month (29 or 30). Throws `RangeError` if the year or month is out of the engine's supported range.
+
+---
+
+## ConversionOptions
+
+```dart
+const ConversionOptions({String calendar = 'uaq'})
+```
+
+| Field | Type | Default | Description |
+| --- | --- | --- | --- |
+| `calendar` | `String` | `'uaq'` | Engine name: `'uaq'` or `'fcna'`, or any registered custom name |
+
+---
+
+## Registry Functions
+
+### registerCalendar
+
+```dart
+void registerCalendar(String name, CalendarEngine engine)
+```
+
+Registers a custom `CalendarEngine` under the given name. The name is case-sensitive. Registering a name that already exists overwrites the previous engine.
+
+### getCalendar
+
+```dart
+CalendarEngine getCalendar(String name)
+```
+
+Retrieves a registered engine by name. Throws `ArgumentError` if the name is not registered.
+
+### listCalendars
+
+```dart
+List<String> listCalendars()
+```
+
+Returns all registered calendar engine names, including the built-in `'uaq'` and `'fcna'`.
+
+---
+
+## CalendarEngine (abstract)
+
+Implement this class to create a custom calendar engine:
+
+```dart
+abstract class CalendarEngine {
+  String get id;
+  HijriDate? toHijri(DateTime date);
+  DateTime? toGregorian(int hy, int hm, int hd);
+  bool isValid(int hy, int hm, int hd);
+  int daysInMonth(int hy, int hm);
+}
+```
+
+---
+
+## Data Exports
+
+### hDatesTable
+
+```dart
+final List<List<int>> hDatesTable
+```
+
+The 184-entry Umm al-Qura reference table covering Hijri years 1318-1501. Each entry contains the Gregorian year, month, and day of the first day of that Hijri year. Used internally by the UAQ engine.
+
+### Month and weekday name lists
+
+| Export | Length | Example |
+| --- | --- | --- |
+| `hmLong` | 12 | `'Muharram'`, `'Safar'`, ... |
+| `hmMedium` | 12 | Abbreviated month names |
+| `hmShort` | 12 | Short month abbreviations |
+| `hwLong` | 7 | `'Al-Ahad'`, `'Al-Ithnayn'`, ... |
+| `hwShort` | 7 | Short weekday names |
+| `hwNumeric` | 7 | Numeric weekday strings |
+
+---
+
+[Home](Home)

.wiki/Home.md

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+# hijri_core
+
+Hijri/Gregorian calendar conversion for Dart and Flutter. Supports Umm al-Qura (UAQ) and FCNA/ISNA calendars. Zero dependencies.
+
+## Quick Start
+
+```dart
+import 'package:hijri_core/hijri_core.dart';
+
+final hijri = toHijri(DateTime.utc(2025, 3, 1));
+print('${hijri!.hy}/${hijri.hm}/${hijri.hd}'); // 1446/9/1
+
+final greg = toGregorian(1446, 9, 1);
+print(greg!.toIso8601String().substring(0, 10)); // 2025-03-01
+```
+
+## Pages
+
+- [API Reference](API-Reference) — Full function and type reference
+- [Calendar Systems](Calendar-Systems) — UAQ and FCNA algorithm details