docs: add built-in providers migration design spec

docs/superpowers/specs/2026-03-28-builtin-providers-design.md

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+# Built-in Providers Migration — Design Spec
+
+## Goal
+
+Move calculator, converter, and system from external `.so` plugins (owlry-plugins repo) to native providers compiled into `owlry-core`. Remove 3 plugin AUR packages (transitional), 4 meta AUR packages (already deleted). Update READMEs for both repos.
+
+## Architecture
+
+The 3 plugins currently use the FFI plugin API (`PluginVTable`, `PluginItem`, etc.) and are loaded as `.so` files by `NativePluginLoader`. As built-in providers, they become native Rust modules inside `owlry-core/src/providers/` implementing the existing `Provider` trait — same as `ApplicationProvider` and `CommandProvider`.
+
+No changes to the plugin system itself. External plugins continue to work via `.so` loading.
+
+## Components
+
+### New modules in owlry-core
+
+- `providers/calculator.rs` — port of owlry-plugin-calculator (231 lines, depends on `meval`)
+- `providers/converter/mod.rs` — port of owlry-plugin-converter entry point
+- `providers/converter/parser.rs` — query parsing (235 lines, no new deps)
+- `providers/converter/units.rs` — unit definitions + conversion (944 lines, no new deps)
+- `providers/converter/currency.rs` — ECB rate fetching (313 lines, depends on `reqwest` blocking + `dirs` + `serde`)
+- `providers/system.rs` — port of owlry-plugin-system (257 lines, no new deps)
+
+### New owlry-core dependencies
+
+- `meval` — math expression evaluation (currently optional behind `lua` feature, make required)
+- `reqwest` with `blocking` feature — ECB currency rate fetching (currently optional behind `lua`, make required)
+- `dirs` — already a dependency
+- `serde`/`serde_json` — already dependencies
+
+### Modified files
+
+- `owlry-core/src/providers/mod.rs` — register the 3 new providers in `ProviderManager`, honor config toggles, classify calculator+converter as dynamic providers
+- `owlry-core/Cargo.toml` — move `meval` and `reqwest` from optional to required
+- `owlry-core/src/config/mod.rs` — add `converter` config toggle (calculator and system already exist)
+
+### Provider classification
+
+- Calculator → dynamic (queried per-keystroke via `query()`)
+- Converter → dynamic (queried per-keystroke via `query()`)
+- System → static (populated at `refresh()`, returns fixed list of actions)
+
+## Provider Type IDs
+
+Built-in providers use `ProviderType::Plugin(String)` with fixed IDs to maintain backward compatibility with the UI highlighting and filter system:
+
+- Calculator: `ProviderType::Plugin("calc".into())`
+- Converter: `ProviderType::Plugin("conv".into())`
+- System: `ProviderType::Plugin("sys".into())`
+
+This ensures the UI's highlighting logic (`matches!(id.as_str(), "calc" | "conv")`) and CSS badge classes (`.owlry-badge-calc`, `.owlry-badge-sys`) continue to work without changes.
+
+## Config
+
+Existing toggles in `[providers]`:
+
+```toml
+[providers]
+calculator = true   # already exists
+system = true       # already exists
+converter = true    # new — add with default true
+```
+
+When a toggle is false, the provider is not registered in `ProviderManager` at startup.
+
+## Currency Conversion
+
+The converter's currency feature uses `reqwest` (blocking) to fetch ECB exchange rates with a 24-hour file cache at `~/.cache/owlry/ecb_rates.json`. If the HTTP fetch fails (no network, timeout), currency conversion silently returns no results — unit conversion still works. This matches current plugin behavior.
+
+## AUR Changes
+
+### Main repo (owlry)
+
+- `aur/owlry-core/PKGBUILD` — bump version
+- Remove `aur/owlry-meta-*` directories (4 dirs, already deleted from AUR)
+
+### Plugins repo (owlry-plugins)
+
+- Remove crates: `owlry-plugin-calculator`, `owlry-plugin-converter`, `owlry-plugin-system`
+- Remove AUR dirs: `aur/owlry-plugin-calculator`, `aur/owlry-plugin-converter`, `aur/owlry-plugin-system` from tracked files
+- Push transitional PKGBUILDs to the 3 AUR repos:
+
+```bash
+pkgname=owlry-plugin-calculator  # (and converter, system)
+pkgver=<last_version>
+pkgrel=99
+pkgdesc="Transitional package — calculator is now built into owlry-core"
+arch=('any')
+depends=('owlry-core>=<new_version>')
+replaces=('owlry-plugin-calculator')
+# No source, no build, no package body
+```
+
+### Conflict prevention
+
+When owlry-core gains built-in calculator/converter/system, users who have the old `.so` plugins installed will have both the built-in provider AND the `.so` plugin active — duplicate results. The daemon should detect this: if a built-in provider ID matches a loaded native plugin ID, skip the native plugin. Add this check in `ProviderManager` when registering native plugins.
+
+## README Updates
+
+### Main repo README
+
+- Package table: remove separate plugin entries for calculator, converter, system — note them as built-in to owlry-core
+- Remove meta package section entirely
+- Update install examples (no need to install calculator/converter/system separately)
+
+### Plugins repo README
+
+- Remove calculator, converter, system from plugin listing
+- Add note that these 3 are built into owlry-core
+
+## Testing
+
+- Port existing plugin tests directly — they test provider logic, not FFI wrappers
+- `cargo test -p owlry-core --lib` covers all 3 new providers
+- Add conflict detection test (built-in provider ID vs native plugin ID)
+- Manual verification: `= 5+3` (calc), `20F` (conv), `20 euro to dollar` (currency), system actions