Move hash fallback logic to cryptobackend

[Quim Muntal (qmuntal)]

This change moves SHA-256, SHA-512, fixed-output SHA-3, SHAKE, and cSHAKE backend/fallback dispatch into cryptobackend. Keeping that decision in one place makes the standard-library integration much closer to upstream and the patches smaller.

Updates #2489

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Copilot review overview

🟡 Changes recommended

The updated crypto/sha256 Sum224 path can still route into an unsupported backend implementation under systemcrypto (missing capability gating), risking runtime panics on Windows/Darwin.

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Review tier: Lite
Findings: 1 · 1

New issues introduced by this change (2)

Severity	Finding
	patches/​0002-Add-crypto-backends.patch — In the sha256.Sum224 patch hunk, the implementation still unconditionally uses…
	cryptobackend/​sha3/​shake.go — This file imports the root cryptobackend package without the conventional boring alias and then…

What changed in this PR

This PR moves SHA-2/SHA-3 (fixed-output), SHAKE, and cSHAKE backend-vs-fallback dispatch logic into github.com/microsoft/go/cryptobackend, replacing scattered capability checks in stdlib packages with generated dispatcher wrappers that centralize support checks, cloning, marshaling, and fallback behavior.

Changes:

• Add generated cryptobackend/{sha256,sha3,sha512} dispatchers (plus SHAKE/cSHAKE helpers) that select backend implementations when supported and fall back otherwise.
• Update stdlib integration (via patches/0002-Add-crypto-backends.patch) to call the new cryptobackend/* wrappers and adjust FIPS/hash unwrap paths accordingly.
• Remove the redundant cryptobackend/hash package and route FIPS approval checks through backend-specific callbacks.

File	Description
patches/​0001-Vendor-external-dependencies.patch	Vendors updated cryptobackend code generation output and removes the vendored cryptobackend/hash package.
patches/​0002-Add-crypto-backends.patch	Updates stdlib crypto packages and internal FIPS plumbing to use cryptobackend for hash dispatch/fallback and unwrap behavior.
cryptobackend/​backend_darwin.go	Adds backend-level fipsApprovedHash callback implementation for Darwin.
cryptobackend/​backend_msgostd.go	Wires crypto/internal/fips140only.BackendApprovedHash to the backend callback under msgostd.
cryptobackend/​backend_openssl.go	Adds backend-level fipsApprovedHash callback implementation for OpenSSL.
cryptobackend/​backend_windows.go	Adds backend-level fipsApprovedHash callback implementation for Windows.
cryptobackend/​backend_test.go	Updates backend tests to reflect removal of the hash subpackage.
cryptobackend/​hash/​hash_darwin.go	Removes the obsolete cryptobackend/hash implementation (Darwin).
cryptobackend/​hash/​hash_openssl.go	Removes the obsolete cryptobackend/hash implementation (OpenSSL).
cryptobackend/​hash/​hash_windows.go	Removes the obsolete cryptobackend/hash implementation (Windows).
cryptobackend/​hash/​nobackend.go	Removes the obsolete cryptobackend/hash stub.
cryptobackend/​internal/​hashgen/​main.go	Adds a code generator that produces native/fallback hash dispatchers for multiple algorithms.
cryptobackend/​sha256/​nobackend.go	Updates SHA-256 backend stubs to match the generated dispatcher structure.
cryptobackend/​sha256/​sha256.go	Adds SHA-256 one-shot sum wrappers that select backend or fallback.
cryptobackend/​sha256/​sha256_darwin.go	Implements backend plumbing and capability checks for SHA-256 on Darwin.
cryptobackend/​sha256/​sha256_openssl.go	Implements backend plumbing and capability checks for SHA-256 on OpenSSL.
cryptobackend/​sha256/​sha256_windows.go	Implements backend plumbing and capability checks for SHA-256 on Windows.
cryptobackend/​sha256/​zsha256.go	Generated SHA-256 hash dispatcher (backend vs fallback) with cloning/marshal support.
cryptobackend/​sha256/​zsha256_msgostd.go	msgostd fallback bindings + constructors + Unwrap.
cryptobackend/​sha256/​zsha256_nomsgostd.go	Non-msgostd panic stubs + constructors.
cryptobackend/​sha3/​init.go	(Integration) Ensures sha3 backend package is pulled in when enabled.
cryptobackend/​sha3/​nobackend.go	Updates SHA-3 backend stubs to match generated dispatcher expectations.
cryptobackend/​sha3/​sha3.go	Adds fixed-output SHA-3 one-shot sum wrappers that select backend or fallback.
cryptobackend/​sha3/​sha3_darwin.go	Implements backend plumbing and capability checks for SHA-3 on Darwin.
cryptobackend/​sha3/​sha3_openssl.go	Implements backend plumbing and capability checks for SHA-3/SHAKE/cSHAKE on OpenSSL.
cryptobackend/​sha3/​sha3_windows.go	Implements backend plumbing and capability checks for SHA-3/SHAKE/cSHAKE on Windows.
cryptobackend/​sha3/​shake.go	Adds SHAKE/cSHAKE dispatch layer (backend vs fallback) and one-shot sum helpers.
cryptobackend/​sha3/​shake_msgostd.go	msgostd fallback bindings for SHAKE/cSHAKE.
cryptobackend/​sha3/​shake_nomsgostd.go	Non-msgostd panic stubs for SHAKE/cSHAKE fallback.
cryptobackend/​sha3/​zsha3.go	Generated SHA-3 hash dispatcher (backend vs fallback) with cloning/marshal support.
cryptobackend/​sha3/​zsha3_msgostd.go	msgostd fallback bindings + constructors + Unwrap.
cryptobackend/​sha3/​zsha3_nomsgostd.go	Non-msgostd panic stubs + constructors.
cryptobackend/​sha512/​nobackend.go	Updates SHA-512 backend stubs to match generated dispatcher structure.
cryptobackend/​sha512/​sha512.go	Adds SHA-512 one-shot sum wrappers that select backend or fallback.
cryptobackend/​sha512/​sha512_darwin.go	Implements backend plumbing and capability checks for SHA-512 on Darwin.
cryptobackend/​sha512/​sha512_openssl.go	Implements backend plumbing and capability checks for SHA-512 on OpenSSL.
cryptobackend/​sha512/​sha512_windows.go	Implements backend plumbing and capability checks for SHA-512 on Windows.
cryptobackend/​sha512/​zsha512.go	Generated SHA-512 hash dispatcher (backend vs fallback) with cloning/marshal support.
cryptobackend/​sha512/​zsha512_msgostd.go	msgostd fallback bindings + constructors + Unwrap.
cryptobackend/​sha512/​zsha512_nomsgostd.go	Non-msgostd panic stubs + constructors.

Files not reviewed (6)

• cryptobackend/sha256/zsha256.go: Generated file
• cryptobackend/sha256/zsha256_msgostd.go: Generated file
• cryptobackend/sha256/zsha256_nomsgostd.go: Generated file
• cryptobackend/sha3/zsha3.go: Generated file
• cryptobackend/sha3/zsha3_msgostd.go: Generated file
• cryptobackend/sha3/zsha3_nomsgostd.go: Generated file

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