Poseidon hash function

[OlivierBBB]

Note

Medium Risk
Compiler and constant-evaluation changes for 𝔽 casts and memory layout affect correctness of generated code; the large Poseidon surface is mostly testdata but increases CI runtime and maintenance burden.

Overview
Adds a Poseidon sponge hash in ZKC (Koalabear, width 16) with parallel u32 and native 𝔽 implementations, static MDS/round constants, and extensive bench/unit/invalid coverage under testdata/zkc/bench/poseidon/.

Strengthens field-element handling: casts and constants are checked against the field modulus in typing and constant evaluation; memory codegen uses native registers for 𝔽 data lines; IncrementalCompiler defaults to KOALABEAR_16; tests gain KOALABEAR_CONFIG plus felt memory/casting and overflow invalid cases.

Renames Flattern → Flatten across Corset source maps, ZKC types/codegen/lowering, and IR term simplifiers (mechanical API spelling fix). Small codegen/parser fixes (e.g. unwritable memory panic text, function AST comment).

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[Copilot]

Pull request overview

This PR adds a Poseidon hash benchmark/test suite for the zkc DSL, including u32-emulated and native felt implementations, and updates compiler support needed for felt-typed memory/static constants. It also performs a repository-wide spelling rename from flattern to flatten.

Changes:

• Adds Poseidon constants, MDS matrices, result tables, implementations, documentation, and zkc benchmark tests.
• Adds felt memory unit tests and compiler/codegen changes for felt memory and constants.
• Renames flattern identifiers/comments to flatten across compiler, IR, and corset packages.

Reviewed changes

Copilot reviewed 95 out of 95 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
pkg/zkc/compiler/codegen/evaluate.go	Adds field-aware constant evaluation paths.
pkg/zkc/compiler/codegen/statement.go	Allows felt data lines for writable memories.
pkg/zkc/compiler/codegen/flatten.go	Renames codegen flatten helper.
pkg/zkc/compiler/codegen/compile.go	Updates callers to renamed flatten helper.
pkg/zkc/compiler/incremental_compiler.go	Defaults incremental compiler field.
pkg/zkc/compiler/parser/parser.go	Fixes error propagation and comment spelling.
pkg/test/zkc_bench_test.go	Wires Poseidon benchmark tests.
pkg/test/zkc_unit_test.go	Wires felt memory unit tests.
testdata/zkc/unit/*felt_memory*	Adds felt memory/static table test cases.
testdata/zkc/bench/poseidon/**	Adds Poseidon implementations, constants, docs, and test fixtures.
pkg/corset/**, pkg/ir/**, pkg/zkc/vm/**, pkg/asm/**	Applies flattern → flatten spelling/identifier cleanup.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[DavePearce]

Ok, there are some problems here. We do not want 70 odd files for a single benchmark!! You need to tidy this up and remove all of the various debugging / test harness stuff. I want these files only:

testdata/zkc/benchmark/poseidon_felt.zkc   <-- contains complete benchmark for felt
testdata/zkc/benchmark/poseidon_felt.accepts
testdata/zkc/benchmark/poseidon_u32.zkc.   <-- contains complete benchmark for u32
testdata/zkc/benchmark/poseidon_u32.accepts

The fact that a README.md is needed to explain what is going on tells me that something is seriously off.