docs: expand LLVM backend ENV vars documentation

Phase 22.x: add NYASH_LLVM_NATIVE_TRACE and expand NYASH_LLVM_BACKEND
- Add auto mode with llvmlite→crate fallback priority
- Add NYASH_LLVM_NATIVE_TRACE=1 for native IR stderr dump
- Clarify each backend option (llvmlite/crate/native)

docs/ENV_VARS.md

@@ -89,14 +89,20 @@ Call/route unified trace (optional)
 - NYASH_DISABLE_NY_COMPILER=1, HAKO_DISABLE_NY_COMPILER=1
 
 LLVM backend selector (builder wrapper)
-- NYASH_LLVM_BACKEND=llvmlite|crate|native
+- NYASH_LLVM_BACKEND=auto|llvmlite|crate|native
   - Selects the backend used by `tools/ny_mir_builder.sh` for `--emit obj|exe`.
-  - Default: `llvmlite` (Python harness `tools/llvmlite_harness.py`).
+  - Default: `auto` (auto-detection: llvmlite優先、fallback to crate if llvmlite unavailable).
+  - `llvmlite`: Python harness `tools/llvmlite_harness.py` (requires llvmlite installed).
   - `crate`: uses `./target/release/ny-llvmc` (build with `cargo build -p nyash-llvm-compiler --release`).
   - `native`: reserved for future Hako-native builder.
   - Linking extras for `--emit exe`: pass via `HAKO_AOT_LDFLAGS` (e.g., `-static`), `ny-llvmc` consumes `--libs`.
   - Note: crate 経路では ny_main の戻り値（i64）がプロセスの終了コードに反映されます（rc mapping）。
 
+- NYASH_LLVM_NATIVE_TRACE=0|1
+  - When 1, dumps the native IR to stderr for debugging.
+  - Used with `native` backend to inspect generated LLVM IR before optimization.
+  - Default OFF; only active when NYASH_LLVM_BACKEND=native is set.
+
 - HAKO_LLVM_CANARY_NORMALIZE=0|1
   - 開発/カナリア専用の正規化スイッチ。`1` のとき、最小の JSON 形状差（`schema_version=1` → `"1.0"`、`blocks.inst` → `instructions`、`const` の `ty/value` 包装）を自動補正してからビルドします。
   - 既定は `0`（無効）。既存ツールの挙動は変わりません。`NYASH_CLI_VERBOSE=1` のとき形状ヒントを `[ny-llvmc/hint]` で出力します。