ported legacy DoubleDouble with tests

AGENTS.md

@@ -25,8 +25,10 @@ Do not claim API completeness or accuracy beyond tested contracts.
 - Use the ignored, repository-local `.hermes/` directory for plans, checklists,
   investigation notes, and handoff context. Create or update notes as useful;
   keep them concise and revalidate them against current files. Optional
-  `.hermes/project-context.md` holds setup and review background. Do not store
-  secrets, force-add this directory, or make builds or tests depend on it.
+  `.hermes/README.md` indexes active notes and `.hermes/project-context.md` holds
+  condensed implementation context. Treat `.hermes/archive/` as historical, not
+  current instructions or status. Do not store secrets, force-add this directory,
+  or make builds or tests depend on it.
 
 ## Working rules
 
@@ -67,10 +69,11 @@ Follow `.editorconfig`, not incidental style in unfinished code.
   Mathematically equivalent formulas can round differently.
 - State and verify algorithm preconditions, especially magnitude ordering for
   quick-sum transforms, normalization assumptions, and overflow/underflow limits.
-- The internal two-component `DoubleDouble` constructor currently does not normalize.
-  Do not assume arbitrary pairs are canonical. Establish the intended normalization,
-  NaN, infinity, and signed-zero contracts before changing construction, equality,
-  hashing, ordering, or classification; keep those operations consistent.
+- The internal two-component `DoubleDouble` constructor does not normalize or
+  validate. Use `FromComponents` for arbitrary pairs and reserve raw construction
+  for proven normalized/canonical results. Preserve the documented normalization,
+  NaN, infinity, and signed-zero contracts across construction, equality, hashing,
+  ordering, and classification; do not change one in isolation.
 - For affected operations, cover cancellation, widely separated magnitudes, zero
   and signed zero, subnormals, extreme finite values, infinities, and NaNs. Check
   intermediate overflow/underflow even when the final result is representable.