[doc] Add enhancement plan for Decimal128 type (#193)

This PR adds a detailed enhancement/audit plan for the
`decimo.decimal128` module, capturing cross-language comparisons,
identified correctness gaps, performance bottlenecks, and a prioritized
execution order.

**Changes:**
- Introduces a comprehensive design/audit document for `Decimal128`,
including a cross-language feature matrix.
- Documents key architectural concerns (notably the 2^96−1 coefficient
bound) and related performance implications.
- Lists suspected correctness/performance issues and proposes a phased
implementation plan.

Author: forfudan

.github/workflows/run_tests.yaml

@@ -2,5 +2,5 @@
 set -e
 
 for f in tests/bigdecimal/*.mojo; do
-    pixi run mojo run -I src -D ASSERT=all --debug-level=line-tables "$f"
+    pixi run mojo run -I src -D ASSERT=all --debug-level=full "$f"
 done

docs/plans/decimal128_enhancement.md

@@ -37,7 +37,7 @@ trap cleanup EXIT
 for f in tests/bigfloat/*.mojo; do
     echo "=== $f ==="
     TMPBIN=$(mktemp /tmp/decimo_test_bigfloat_XXXXXX)
-    pixi run mojo build -I src --debug-level=line-tables \
+    pixi run mojo build -I src --debug-level=full \
         -Xlinker -L./"$WRAPPER_DIR" -Xlinker -ldecimo_gmp_wrapper \
         -o "$TMPBIN" "$f"
     DYLD_LIBRARY_PATH="./$WRAPPER_DIR" LD_LIBRARY_PATH="./$WRAPPER_DIR" "$TMPBIN"

tests/test_bigdecimal.sh

@@ -2,5 +2,5 @@
 set -e
 
 for f in tests/bigint/*.mojo; do
-    pixi run mojo run -I src -D ASSERT=all --debug-level=line-tables "$f"
+    pixi run mojo run -I src -D ASSERT=all --debug-level=full "$f"
 done

tests/test_bigfloat.sh

@@ -2,5 +2,5 @@
 set -e
 
 for f in tests/bigint10/*.mojo; do
-    pixi run mojo run -I src -D ASSERT=all --debug-level=line-tables "$f"
+    pixi run mojo run -I src -D ASSERT=all --debug-level=full "$f"
 done

tests/test_bigint.sh

@@ -2,5 +2,5 @@
 set -e
 
 for f in tests/biguint/*.mojo; do
-    pixi run mojo run -I src -D ASSERT=all --debug-level=line-tables "$f"
+    pixi run mojo run -I src -D ASSERT=all --debug-level=full "$f"
 done

tests/test_bigint10.sh

@@ -3,7 +3,7 @@ set -e  # Exit immediately if any command fails
 
 # ── Unit tests ─────────────────────────────────────────────────────────────
 for f in tests/cli/*.mojo; do
-    pixi run mojo run -I src -I src/cli -D ASSERT=all --debug-level=line-tables "$f"
+    pixi run mojo run -I src -I src/cli -D ASSERT=all --debug-level=full "$f"
 done
 
 # ── Integration tests (exercise the compiled binary) ───────────────────────

tests/test_biguint.sh

@@ -2,5 +2,5 @@
 set -e
 
 for f in tests/decimal128/*.mojo; do
-    pixi run mojo run -I src -D ASSERT=all --debug-level=line-tables "$f"
+    pixi run mojo run -I src -D ASSERT=all --debug-level=full "$f"
 done

tests/test_cli.sh

@@ -2,5 +2,5 @@
 set -e
 
 for f in tests/toml/*.mojo; do
-    pixi run mojo run -I src -D ASSERT=all --debug-level=line-tables "$f"
+    pixi run mojo run -I src -D ASSERT=all --debug-level=full "$f"
 done