Add Rust implementation of SCFG decoder

Rust port of the Ruby prototype decoder with performance
optimizations for real Hiero-style grammars:

- Rule indexing by first terminal/NT symbol for fast lookup
- Chart symbol interning (u16 IDs) instead of string hashing
- Passive chart index by (symbol, left) for direct right-endpoint lookup
- Items store rule index instead of cloned rule data

Includes CKY+ parser, chart-to-hypergraph conversion, Viterbi
decoding, derivation extraction, and JSON hypergraph I/O.

Self-filling step in parse uses grammar lookup (not just
remaining active items) to handle rules that were consumed
during the parse loop or skipped by the has_any_at optimization.

Produces identical output to the Ruby prototype on all test examples.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 39 insertions, 0 deletions

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+# odenwald
+
+Rust implementation of the Odenwald SCFG (synchronous context-free grammar) machine translation decoder.
+
+## Build
+
+```
+cargo build --release
+```
+
+## Usage
+
+```
+odenwald -g <grammar> -w <weights> [-i <input>] [-l] [-p]
+```
+
+- `-g, --grammar` — grammar file (required)
+- `-w, --weights` — weights file (required)
+- `-i, --input` — input file (default: stdin)
+- `-l, --add-glue` — add glue rules
+- `-p, --add-pass-through` — add pass-through rules
+
+Output: `translation ||| log_score` per input line.
+
+## Examples
+
+```
+cargo run -- -g ../example/toy/grammar -w ../example/toy/weights.toy -i ../example/toy/in -l
+# → i saw a small shell ||| -0.5
+
+cargo run -- -g ../example/toy-reorder/grammar -w ../example/toy-reorder/weights.toy -i ../example/toy-reorder/in -l
+# → he reads the book ||| -1.5
+```
+
+## Tests
+
+```
+cargo test
+```