feat(which-key): add passive rendering widget crate with multi-column grid layout

Decoupled from evil-keys — accepts generic KeyHint data from any source.
Ports which-key.nvim's dimension calculation and column-major grid algorithm.
Features: unicode-aware truncation, multi-level sorting (group/alphanum/natural/case),
configurable styling via builder pattern, auto-sizing with Position-based placement.

76 tests covering dim() (31 ported from nvim), grid layout, truncation, padding, and sorting.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/claude-agent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/which-key/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "which-key"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+ratatui = "0.29"
+unicode-width = "0.2"
+
+[dev-dependencies]
+proptest = "1.4"
+insta = "1.40.0"