feat(normalized): Phase 141 P1.5 - external env inputs + KnownIntrinsic SSOT

## Task B: External env input bug fix (Priority 1)

Fix: Suffix normalization couldn't access prefix-built local variables

**Problem**: `s.length()` failed because 's' (from prefix `s = "abc"`) was
not in available_inputs during suffix normalization.

**Root cause**: `AvailableInputsCollectorBox::collect()` only collected
function params and CapturedEnv, missing `builder.variable_map`.

**Solution**: Add `prefix_variables` parameter with 3-source merge:
1. Function params (highest priority)
2. Prefix variables (medium priority - NEW)
3. CapturedEnv (lowest priority)

**Changed files**:
- src/mir/control_tree/normalized_shadow/available_inputs_collector.rs
- src/mir/builder/control_flow/normalization/execute_box.rs
- src/mir/builder/control_flow/joinir/patterns/policies/normalized_shadow_suffix_router_box.rs
- src/mir/builder/control_flow/joinir/routing.rs
- src/mir/builder/stmts.rs
- src/mir/control_tree/normalized_shadow/dev_pipeline.rs
- docs/development/current/main/design/normalized-expr-lowering.md (Available Inputs SSOT section)

**Tests**: 3 new unit tests (prefix merge, priority order)

## Task A: KnownIntrinsic SSOT化 (Priority 2)

Eliminate string literal scattered matching by centralizing to registry.

**Problem**: Adding new intrinsics required editing if/match chains with
hard-coded string literals (`if method == KnownIntrinsic::Length0.method_name()`).

**Solution**: Create `KnownIntrinsicRegistryBox` as SSOT:
- `lookup(method, arity) -> Option<KnownIntrinsic>`
- `get_spec(intrinsic) -> KnownIntrinsicSpec`
- Adding new intrinsics now requires: (1) enum variant, (2) registry entry only

**Changed files**:
- src/mir/control_tree/normalized_shadow/common/known_intrinsics.rs (NEW)
- src/mir/control_tree/normalized_shadow/common/expr_lowerer_box.rs
- src/mir/control_tree/normalized_shadow/common/expr_lowering_contract.rs (deprecated methods removed)
- src/mir/control_tree/normalized_shadow/common/mod.rs
- docs/development/current/main/design/normalized-expr-lowering.md (Known Intrinsic SSOT section)

**Impact**: ~30% code reduction in intrinsic matching logic

## Task C: Better diagnostics (Priority 3)

Add `OutOfScopeReason::IntrinsicNotWhitelisted` for precise diagnostics.

**Changed files**:
- src/mir/control_tree/normalized_shadow/common/expr_lowering_contract.rs (enum variant)
- src/mir/control_tree/normalized_shadow/common/expr_lowerer_box.rs (diagnostic logic)

## Verification

✅ Build: `cargo build --release` - PASS
✅ Phase 97 regression: next_non_ws LLVM EXE - PASS
✅ Phase 131: loop(true) break-once VM - PASS
✅ Phase 136: return literal VM - PASS
✅ Phase 137: return x+2 VM - PASS

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/normalization/execute_box.rs

@@ -16,6 +16,7 @@ use crate::ast::ASTNode;
 use crate::mir::builder::MirBuilder;
 use crate::mir::ValueId;
 use super::plan::{NormalizationPlan, PlanKind};
+use std::collections::BTreeMap;
 
 /// Box-First: Execute normalization plan
 pub struct NormalizationExecuteBox;
@@ -23,6 +24,8 @@ pub struct NormalizationExecuteBox;
 impl NormalizationExecuteBox {
     /// Execute a normalization plan
     ///
+    /// ## Phase 141 P1.5: Added prefix_variables parameter
+    ///
     /// Returns:
     /// - Ok(value_id): Successfully executed, returns result value
     /// - Err(_): Lowering or merge failed
@@ -32,6 +35,7 @@ impl NormalizationExecuteBox {
         remaining: &[ASTNode],
         func_name: &str,
         debug: bool,
+        prefix_variables: Option<&BTreeMap<String, ValueId>>,
     ) -> Result<ValueId, String> {
         let trace = crate::mir::builder::control_flow::joinir::trace::trace();
 
@@ -57,20 +61,23 @@ impl NormalizationExecuteBox {
 
         match &plan.kind {
             PlanKind::LoopOnly => {
-                Self::execute_loop_only(builder, remaining, func_name, debug)
+                Self::execute_loop_only(builder, remaining, func_name, debug, prefix_variables)
             }
             PlanKind::LoopWithPost { .. } => {
-                Self::execute_loop_with_post(builder, plan, remaining, func_name, debug)
+                Self::execute_loop_with_post(builder, plan, remaining, func_name, debug, prefix_variables)
             }
         }
     }
 
     /// Execute Phase 131: Loop-only pattern
+    ///
+    /// ## Phase 141 P1.5: Added prefix_variables parameter
     fn execute_loop_only(
         builder: &mut MirBuilder,
         remaining: &[ASTNode],
         func_name: &str,
         debug: bool,
+        prefix_variables: Option<&BTreeMap<String, ValueId>>,
     ) -> Result<ValueId, String> {
         use crate::ast::Span;
         use crate::mir::control_tree::normalized_shadow::available_inputs_collector::AvailableInputsCollectorBox;
@@ -92,8 +99,8 @@ impl NormalizationExecuteBox {
 
         let tree = StepTreeBuilderBox::build_from_ast(&loop_ast);
 
-        // Collect available inputs
-        let available_inputs = AvailableInputsCollectorBox::collect(builder, None);
+        // Collect available inputs (Phase 141 P1.5: with prefix variables)
+        let available_inputs = AvailableInputsCollectorBox::collect(builder, None, prefix_variables);
 
         if debug {
             trace.routing(
@@ -141,12 +148,15 @@ impl NormalizationExecuteBox {
     }
 
     /// Execute Phase 132-133: Loop + post assignments + return
+    ///
+    /// ## Phase 141 P1.5: Added prefix_variables parameter
     fn execute_loop_with_post(
         builder: &mut MirBuilder,
         plan: &NormalizationPlan,
         remaining: &[ASTNode],
         func_name: &str,
         debug: bool,
+        prefix_variables: Option<&BTreeMap<String, ValueId>>,
     ) -> Result<ValueId, String> {
         use crate::mir::control_tree::normalized_shadow::available_inputs_collector::AvailableInputsCollectorBox;
         use crate::mir::control_tree::normalized_shadow::StepTreeNormalizedShadowLowererBox;
@@ -158,8 +168,8 @@ impl NormalizationExecuteBox {
         let suffix = &remaining[..plan.consumed];
         let step_tree = StepTreeBuilderBox::build_from_block(suffix);
 
-        // Collect available inputs
-        let available_inputs = AvailableInputsCollectorBox::collect(builder, None);
+        // Collect available inputs (Phase 141 P1.5: with prefix variables)
+        let available_inputs = AvailableInputsCollectorBox::collect(builder, None, prefix_variables);
 
         if debug {
             trace.routing(