refactor(cleanup): Large-scale dead code elimination (Task 5)

Removed 966 lines of dead code identified by Rust compiler warnings.
4 files completely deleted, multiple pattern implementations cleaned up.

**Files Completely Deleted (4)**:
- src/mir/builder/control_flow/joinir/api/entry.rs (15 lines)
- src/mir/builder/control_flow/joinir/api/pipeline_contracts.rs (17 lines)
- src/mir/builder/control_flow/joinir/api/receiver.rs (21 lines)
- src/mir/builder/control_flow/joinir/patterns/pattern2_steps/promote_step_box.rs (33 lines)

**Pattern 7 (SplitScan) - 423 lines deleted**:
- Removed: can_lower(), lower(), cf_loop_pattern7_split_scan_impl()
- Removed helper functions: contains_methodcall(), contains_variable_step(), contains_push()
- Removed test code
- Kept: extract_split_scan_plan() (used by router.rs)

**Pattern 8 (BoolPredicate) - 222 lines deleted**:
- Removed: cf_loop_pattern8_bool_predicate_impl() (impl MirBuilder block)
- Kept: can_lower(), lower() (used by router.rs)

**Pattern 5 (InfiniteEarlyExit) - 95 lines deleted**:
- Removed: count_breaks_and_continues(), validate_continue_position(), validate_break_pattern()

**Pattern 6 (ScanWithInit) - 110 lines deleted**:
- Removed: contains_methodcall(), is_const_step_pattern()
- Removed 4 test functions

**JoinIR API - 53 lines deleted**:
- Removed module: api/entry.rs
- Removed module: api/pipeline_contracts.rs
- Removed module: api/receiver.rs

**EdgeCFG API - 30 lines deleted**:
- EdgeStub: Removed new(), with_target() methods

**Statistics**:
- Total lines deleted: 966
- Files deleted: 4
- Dead code warnings eliminated: Multiple pattern implementations

Build Status: ✅ Release build successful (0 errors)

🤖 Generated with Claude Code

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

src/mir/builder/control_flow/joinir/api/entry.rs

@@ -1,14 +0,0 @@
-//! Entry function helpers (SSOT)
-
-use crate::mir::join_ir::{JoinFunction, JoinModule};
-
-/// Extract entry function from JoinModule (SSOT).
-///
-/// Priority: `join_module.entry` → fallback to `"main"`.
-pub(in crate::mir::builder) fn get_entry_function<'a>(
-    join_module: &'a JoinModule,
-    context: &str,
-) -> Result<&'a JoinFunction, String> {
-    // Re-exported from patterns/common as the SSOT entry point.
-    super::super::patterns::common::get_entry_function(join_module, context)
-}

src/mir/builder/control_flow/joinir/api/mod.rs

@@ -8,7 +8,3 @@
 //! - Prefer SSOT helpers over ad-hoc logic in patterns.
 //! - Avoid guessing (order/layout/name) in callers; callers pass explicit intent.
 //! - Keep this module thin: mostly wrappers/re-exports with clear naming.
-
-pub(in crate::mir::builder) mod entry;
-pub(in crate::mir::builder) mod pipeline_contracts;
-pub(in crate::mir::builder) mod receiver;

src/mir/builder/control_flow/joinir/api/pipeline_contracts.rs

@@ -1,16 +0,0 @@
-//! Pipeline contract helpers (SSOT)
-
-use crate::mir::join_ir::lowering::inline_boundary::JoinInlineBoundary;
-use crate::mir::join_ir::JoinModule;
-
-/// Run all JoinIR pipeline contract checks (SSOT).
-///
-/// This is the preferred entry point for patterns that need to validate boundary
-/// assumptions before bridge/merge.
-pub(in crate::mir::builder) fn run_all_pipeline_checks(
-    join_module: &JoinModule,
-    boundary: &JoinInlineBoundary,
-) -> Result<(), String> {
-    super::super::merge::contract_checks::run_all_pipeline_checks(join_module, boundary)
-}
-

src/mir/builder/control_flow/joinir/api/receiver.rs

@@ -1,20 +0,0 @@
-//! Receiver helpers (SSOT)
-//!
-//! Motivation: surface syntax uses `this` / `me`, but the MIR builder's `variable_map`
-//! registers the receiver under a specific key. Patterns must not guess that key.
-
-use crate::mir::builder::MirBuilder;
-use crate::mir::ValueId;
-
-/// Return the host ValueId for `me` receiver (SSOT).
-///
-/// Patterns should use this instead of hardcoding `"me"` / `"this"`.
-pub(in crate::mir::builder) fn get_me_host_id(builder: &MirBuilder, context: &str) -> Result<ValueId, String> {
-    builder
-        .variable_ctx
-        .variable_map
-        .get("me")
-        .copied()
-        .ok_or_else(|| format!("[{}] receiver 'me' not found in variable_map", context))
-}
-