docs: add MIR13 mode doc and set PHI-off as default; bridge lowering split (if/loop/try); llvmlite resolver stabilization; curated runner default PHI-off; refresh CURRENT_TASK.md

docs/reference/mir/phi_invariants.md

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+# MIR PHI Invariants
+
+Scope: Builder/Bridge, PyVM, llvmlite (AOT)
+
+Goal: Ensure deterministic PHI formation at control-flow merges so that
+PyVM and LLVM backends agree for nested short-circuit, loop-if merges,
+and chained ternary expressions.
+
+Invariants
+- If-merge ordering: Record incoming as [then, else] in this order when
+  both branches reach the merge. When a branch is structurally absent,
+  synthesize a carry-over from the pre-merge value.
+- Loop latch snapshot: The latch (backedge) snapshot must be taken after
+  per-iteration merges (i.e., after any phi binding for variables assigned
+  in the loop body or nested if). Builder must bind the merged value to the
+  loop-carried variable map before capturing the end-of-body state.
+- Self-carry handling: A PHI with self-carry is allowed only when there is
+  at least one non-self incoming. At finalize, map self-carry to the most
+  recent non-self source visible at the predecessor end.
+
+Representative Cases
+- Nested short-circuit: `a && (b || c)` with selective assignments in nested
+  branches. Expect single-eval per operand and deterministic merge order.
+- Loop + if merge: A running sum updated in only one branch inside a while
+  loop. Expect the latch to capture the phi-merged value, not a pre-merge
+  temporary.
+- Chained ternary: `cond1 ? (cond2 ? x : y) : z`. Expect linearized branches
+  with merge ordering preserved at each join.
+
+Diagnostics
+- Enable `NYASH_LLVM_TRACE_PHI=1` to record per-block snapshots and PHI
+  wiring in the LLVM path.
+- Bridge verifier may allow `verify_allow_no_phi()` in PHI-off mode, but
+  the invariants above still apply to resolver synthesis order.
+