fix(codegen,runtime): entry-initialize boxed slots — eliminate undef box-pointer operands (#4926)

[proggeramlug]

Fixes #4926 — the deferred source bug behind the #4898 react-reconciler SIGBUS (runtime gate shipped in #4902).

Root cause

The boxed-var Stmt::Let path hoisted the slot alloca to the entry block (so it dominates all uses), but the store of the box pointer into that slot only runs where the Let executes. Any boxed read/write on a control-flow path that skips the Let — switch fallthrough into a later case, sibling-branch closure capture — loaded an uninitialized slot. LLVM folds that load to undef, and regalloc substitutes whatever register happens to be live. Under typed-feedback (#854) density (react-reconciler emits ~2500 register_site calls in one minified function), that live value was the read-only "object_get_by_name_guard" cstring pointer → js_box_set wrote into __TEXT.__cstring → SIGBUS. PreallocateBoxes (#569) had the same non-dominating-store shape when nested inside If/Try/Labeled bodies.

Fix

• codegen (stmt/let_stmt.rs, stmt/mod.rs): entry-initialize every boxed slot with TAG_UNDEFINED via entry_allocas_push_store — exactly what the non-boxed Let path already did. Skipped-init paths now read a defined sentinel that even the structural checks alone reject deterministically; no more undef, no more regalloc roulette.
• runtime (box.rs): js_box_get on an unregistered pointer returns undefined instead of bare NaN — the JS value of a read-before-initialization variable (Perry has no TDZ). TAG_UNDEFINED is a quiet-NaN bit pattern, so numeric consumers behave exactly as before; typeof/null-checks now see undefined.
• e2e test (crates/perry/tests/issue_4926_boxed_slot_skipped_init.rs): switch-case skipped-init boxed read/write must be deterministic undefined, taken paths unchanged.

Verification

• Mechanism repro (switch-case let captured+mutated by a closure, entered at the later case): pre-fix PERRY_DEBUG=1 showed js_box_get: invalid box pointer 0x1 (undef materialized as garbage) and typeof x printed number; post-fix the pointer is exactly the sentinel 0x7ffc000000000001 and typeof x prints undefined.
• react-reconciler (rr_min.tsx from react-reconciler 0.33 init crashes under Perry — SIGBUS on import (standalone) / 'undefined is not iterable' in createReconciler (in-ink); blocks all custom React renderers #4898): the stray js_box_set operand collapses from arbitrary __TEXT garbage (0x1016e9360) to the deterministic sentinel — proving the rr stray call was this exact mechanism and is now defined behavior end to end. createReconciler({...full hostconfig}) still builds a working reconciler.
• Zero regressions: 225-file test_gap_* sweep vs node --experimental-strip-types — failure sets byte-identical between baseline (clean main) and fixed builds (190 pass / 35 pre-existing, the only output diffs being ASLR addresses and thread IDs in panic backtraces). Full perry-runtime suite green single-threaded (1013 passed; the 1 failure, date::tests::test_full_year_setters_revive_invalid_date_only, fails on main too). cargo fmt clean; all touched files under the 2000-line gate.

No version bump / CHANGELOG entry per maintainer convention (folded in at merge).