Initial commit: Rust calendar utility

plugins/holiday_highlighter/README.md

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+# Holiday Highlighter Plugin
+
+Plugin for `cal` that adds holiday highlighting via isdayoff.ru API.
+
+## Features
+
+- **Automatic country detection** from system locale
+- **Data caching** to reduce API requests
+- **Multiple country support**: Russia, Belarus, Kazakhstan, USA, Uzbekistan, Turkey, Latvia
+- **Day type data**: working days, weekends, shortened days, official holidays
+
+## Building
+
+```bash
+# Build entire workspace
+cargo build --release --workspace
+
+# Plugin only
+cargo build --release -p holiday_highlighter
+```
+
+## Installation
+
+### User-local
+
+```bash
+mkdir -p ~/.local/lib/cal/plugins
+cp target/release/libholiday_highlighter.so ~/.local/lib/cal/plugins/
+```
+
+### System-wide
+
+```bash
+sudo mkdir -p /usr/lib/cal/plugins
+sudo cp target/release/libholiday_highlighter.so /usr/lib/cal/plugins/
+```
+
+## Usage
+
+```bash
+# Highlight holidays for current country
+cal -H
+
+# Year with holidays
+cal -y -H
+
+# Three months with holidays
+cal -3 -H
+```
+
+## Supported countries
+
+| Code | Country | Auto-detect locales |
+|------|---------|---------------------|
+| RU | Russia | ru_RU, ru_BY, ru_KZ, ru_UZ, ru_LV |
+| BY | Belarus | be_BY, ru_BY |
+| KZ | Kazakhstan | kk_KZ, ru_KZ |
+| US | USA | en_US, en |
+| UZ | Uzbekistan | uz_UZ, ru_UZ |
+| TR | Turkey | tr_TR |
+| LV | Latvia | lv_LV, ru_LV |
+
+## API
+
+The plugin uses [isdayoff.ru API](https://isdayoff.ru/):
+
+### Monthly request
+
+```
+GET https://isdayoff.ru/api/getdata?year=2026&month=01&pre=1
+```
+
+### Yearly request
+
+```
+GET https://isdayoff.ru/api/getdata?year=2026&pre=1
+```
+
+Parameter `pre=1` includes pre-holiday shortened days information.
+
+## Data format
+
+Each character in the response represents a day of the month:
+
+| Character | Day type | Description |
+|-----------|----------|-------------|
+| `0` | Working | Regular working day |
+| `1` | Weekend | Saturday or Sunday |
+| `2` | Shortened | Pre-holiday shortened day |
+| `8` | Holiday | Official public holiday |
+
+## Environment variables
+
+| Variable | Description |
+|----------|-------------|
+| `LC_ALL` | Priority locale for country detection |
+| `LC_TIME` | Locale for country detection |
+| `LANG` | Fallback locale for country detection |