all: drop support for Go 1.18 through 1.24

[hajimehoshi]

What issue is this addressing?

Updates #468

What type of issue is this addressing?

feature

What this PR does | solves

Drops support for Go 1.18 through 1.24, raising the minimum to Go 1.25.

The CI matrix exercised every Go release back to 1.18, which makes test
runs long. That cost only grows as more targets are added (e.g. a future
windows/arm64). Narrowing the supported range also lets us drop
version-gated workarounds, reducing maintenance burden.

Changes:

• go.mod: language version 1.18 → 1.25.0.
• CI (.github/workflows/test.yml): trim the matrix to the two most
  recent Go releases (1.25.x, 1.26.x), and drop the now-always-true
  !startsWith(matrix.go, '1.18'/'1.19') guards on the race steps.
• Remove compatibility shims that only existed for older toolchains:
  • internal/xreflect (polyfilled reflect.TypeAssert) — callers now
    use the stdlib reflect.TypeAssert directly.
  • the objc clone helper — replaced by slices.Clone.
• Modernize the remaining code to the new baseline: integer range
  loops, reflect.TypeFor, and an iter.Seq-based field iterator in
  the struct tests.

Verified locally: go vet, gofmt -s, go build (native +
cross-compile to linux/windows/freebsd), and the full test suite
(CGO_ENABLED=1) all pass.

---

Authored by Claude (Claude Code), on behalf of @hajimehoshi.

[Copilot]

Pull request overview

This PR raises the minimum supported Go version to 1.25 to reduce CI matrix runtime and remove now-unnecessary compatibility shims, while modernizing code to use newer stdlib/runtime features.

Changes:

• Bump go.mod language version to Go 1.25 and trim CI to Go 1.25.x/1.26.x.
• Remove internal/xreflect shim usage and switch call sites to reflect.TypeAssert / reflect.TypeFor.
• Modernize loops and test helpers (integer range loops; iter.Seq-based struct-field iteration; slices.Clone in objc runtime).

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
go.mod	Raises module language version baseline to Go 1.25.
.github/workflows/test.yml	Trims CI Go versions and simplifies race-step guards.
internal/xreflect/reflect_go124.go	Removes pre-Go1.25 TypeAssert polyfill.
internal/xreflect/reflect_go125.go	Removes Go1.25 TypeAssert wrapper.
func.go	Switches to reflect.TypeFor / reflect.TypeAssert now that Go 1.25 is required.
syscall_unix.go	Replaces reflect.TypeOf(CDecl{}) with reflect.TypeFor[CDecl]().
struct_arm64.go	Modernizes a few reflection/type usages and converts one loop to integer-range form.
struct_test.go	Uses iter.Seq for struct-field iteration and relies on new range-var semantics.
syscall_bench_test.go	Uses integer range loops in benchmark helper.
func_test.go	Uses integer range loops in concurrency test.
callback_test.go	Uses integer range loops in callback test.
objc/objc_runtime_darwin.go	Removes xreflect, uses reflect.TypeAssert / reflect.TypeFor, and replaces custom clone with slices.Clone.
objc/objc_block_darwin.go	Removes xreflect and uses reflect.TypeAssert / reflect.TypeFor.
objc/objc_runtime_darwin_test.go	Updates examples to use reflect.TypeFor.
examples/objc/main_darwin.go	Updates example to use reflect.TypeFor.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[TotallyGamerJet]

LGTM