feat: add Khipro compositional input method

[qomarhsn]

Introduce a third input-method category alongside phonetic and fixed: "compositional", which turns a roman key sequence into Bengali through a deterministic engine. Khipro is the first such layout, driven by an embedded m17n .mim spec.

• src/compositional/: CompositionalEngine trait, Method impl (tape replay
  • backspace), and a pure-Rust port of the khipro library's s-expression parser, spec AST, and FSM interpreter (mim/).
• config: add LayoutMethod + compositional_suggestion setting; recognize the built-in "khipro" layout name.
• context: dispatch to CompositionalMethod via layout_method().
• ffi: riti_config_set_compositional_suggestion (+ riti.h declaration).

The first candidate is the exact Khipro composition; dictionary/emoji suggestions follow, as with the fixed method. The candidate-window auxiliary text shows the raw roman keystrokes.

Vendored data (data/khipro/), MIT licensed:

• bn-khipro.mim (c) 2024 rank_coder - embedded at runtime
• khipro-testcases.csv - used only by the #[cfg(test)] conformance test

[mominul]

@qomarhsn Hi!

Thanks for the pull request! But have you checked the OpenBangla/OpenBangla-Keyboard#426 ticket beforehand? We already have Khipro support in the Okkhor crate. The remaining work was about generating suggestions for Khipro, which I have done initially in the khipro branch in the upodesh repo.

Can you give me the rationale why we should proceed to have a .mim based layout system?