Add explicit fallback to array reduction; add error value

[ducky64]

Changes the IR and compiler to define the empty array output of array reduction operations in the frontend, allowing better type propagation (especially disambiguating float 0 and range 0 for sums).

Changes the error handling to generate an error value instead of crashing the compiler, allowing a partial compile instead of a unhelpful stack trace.

In the compiler, moves the param errors into the param value, while tracking overassigns separately because of the immutability of the param value system.

Also tightens up what reduction operations are allowed in the array subtypes.

Resolves #223

[Copilot]

Pull request overview

This PR updates the EDG IR and both the Python frontend + Scala compiler to (1) make unary set/reduction operations define an explicit result for empty sets/arrays (improving type propagation), and (2) represent compiler/evaluation failures as an error value that can propagate, enabling partial compilation instead of hard failures (resolves #223).

Changes:

• Extend the IR with ValueLit.error / ErrorLit and propagate errors through the compiler as a first-class value.
• Add empty_value to UnarySetExpr and update frontend bindings + compiler evaluation/visitors to use explicit empty-set fallbacks.
• Rework constant propagation error handling to stop propagation on errors and track over-assignments separately; export parameter errors in compiled design output.

Reviewed changes

Copilot reviewed 32 out of 33 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
proto/edgir/lit.proto	Adds ErrorLit and ValueLit.error oneof arm for error literals.
proto/edgir/expr.proto	Adds UnarySetExpr.empty_value to define empty-set results (esp. reductions).
edg/hdl_server/main.py	Bumps Python-side proto version constant to 11.
edg/edgir/lit_pb2.pyi	Regenerated stubs to include ErrorLit / ValueLit.error.
edg/edgir/lit_pb2.py	Regenerated Python protobuf code for new literal types.
edg/edgir/expr_pb2.pyi	Regenerated stubs to include UnarySetExpr.empty_value.
edg/edgir/expr_pb2.py	Regenerated Python protobuf code for UnarySetExpr.empty_value.
edg/edgir/init.py	Adds Python ErrorValue and decodes ValueLit.error to it.
edg/core/test_simple_expr_eval.py	Adds test coverage for overassign producing an error value when ignore_errors=True.
edg/core/test_link.py	Updates expected constraint protos to include unary_set.empty_value.
edg/core/test_compiled_design_export.py	Adds export test ensuring param errors surface in exported JSON model.
edg/core/test_block_portvector.py	Updates expected SUM constraint to include empty_value.
edg/core/CompiledDesignExport.py	Adds CompiledParam.error field and maps ErrorValue into export output.
edg/core/Binding.py	Extends UnarySetOpBinding to populate empty_value into expr protos.
edg/core/ArrayExpr.py	Moves reductions into typed array subclasses and supplies explicit empty fallbacks.
edg/core/Array.py	Sets empty_value for FLATTEN to an explicitly empty array literal.
compiler/src/test/scala/edg/compiler/TunnelExportTest.scala	Updates test IR to pass emptyValue to UnarySetOp.
compiler/src/test/scala/edg/compiler/ExprEvaluateTest.scala	Updates unary-set evaluation tests to include emptyValue.
compiler/src/test/scala/edg/compiler/ExprEvaluatePartialTest.scala	Updates partial-eval tests for new UnarySetOp signature.
compiler/src/test/scala/edg/compiler/ConstPropAssignTest.scala	Adds tests for overassign errors producing ErrorValue and not over-propagating.
compiler/src/test/scala/edg/compiler/ConstPropArrayTest.scala	Updates array reduction tests with explicit emptyValue args.
compiler/src/test/scala/edg/compiler/CompilerEvaluationTest.scala	Updates compiler evaluation tests with explicit reduction empty fallbacks.
compiler/src/main/scala/edg/util/DependencyGraph.scala	Adds stop option to setValue to halt propagation without crashing.
compiler/src/main/scala/edg/ExprBuilder.scala	Updates UnarySetOp builder signature and adds literal Error helper.
compiler/src/main/scala/edg/compiler/ValueExprMap.scala	Updates visitor signature for unary-set mapping to include emptyValue.
compiler/src/main/scala/edg/compiler/ExprValue.scala	Makes ErrorValue IR-representable; supports literal conversion + optional messages.
compiler/src/main/scala/edg/compiler/ExprToString.scala	Adds rendering for error literals; updates unary-set map signature.
compiler/src/main/scala/edg/compiler/ExprEvaluatePartial.scala	Updates partial evaluation to evaluate/track refs in both vals and emptyValue.
compiler/src/main/scala/edg/compiler/ExprEvaluate.scala	Uses explicit emptyValue for empty arrays; converts prior hard errors to ErrorValue(Some(...)).
compiler/src/main/scala/edg/compiler/DesignRefsValidate.scala	Collects references from both unary-set vals and emptyValue.
compiler/src/main/scala/edg/compiler/ConstProp.scala	Reworks error handling to store ErrorValue in graph and track overassigns separately.
compiler/src/main/scala/edg/compiler/Compiler.scala	Bumps expected proto version; updates FLATTEN nodes to provide an empty array fallback.

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