add new AGENTS invariant

invariants/PARALLELISM_AND_RUNTIME_INVARIANT.md

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+# Parallelism and Runtime Invariant
+
+Compile-time optimization must not trade away execution parallelism or worsen
+theoretical runtime behavior. The absence of a precise runtime measurement does
+not make such a trade acceptable.
+
+In particular, a compile-time optimization must not:
+
+- serialize work that can execute independently;
+- coarsen lane, core, batch, or scheduling granularity in a way that reduces
+  available execution parallelism;
+- increase the theoretical runtime critical path, instruction count, memory
+  traffic, synchronization, or required copies merely to reduce compiler work;
+- merge independently scheduled work when doing so may reduce concurrency.
+
+Compiler representations may share analysis, planning, or code-generation
+work only when the emitted execution retains the same independent work,
+schedule semantics, and available parallelism. If runtime impact cannot be
+measured precisely, it must be established from the transformation's semantics;
+uncertainty is not permission to accept a possible runtime regression.