fix(msl): vertex stage_in for struct args, metal::discard_fragment namespace (v0.14.4)

* test: add coverage tests for MSL/GLSL backends, IR validator

- msl/expressions_test.go: MSL expression generation tests
- msl/statements_test.go: MSL statement generation tests
- msl/types_test.go: MSL type mapping, entry point tests
- glsl/expressions_test.go: GLSL type/expression/statement tests
- ir/validate_expression_test.go: comprehensive expression/type validation
- Removed old validate_test.go (superseded by new comprehensive tests)

* fix: remove incorrect gl_GlobalInvocationID assertion in GLSL compute test

* fix(msl): vertex stage_in for struct args, metal::discard_fragment namespace

- Remove fragment-only gate in writeEntryPointInputStruct, allowing
  vertex shaders with struct-typed inputs (e.g. fn vs_main(in: VertexInput))
  to generate synthesized _Input struct with [[attribute(N)]] members
- Use member binding-based attribute generation (locationInputAttribute,
  builtinInputAttribute) instead of hardcoded position/user attributes
- Add metal:: namespace prefix to discard_fragment() call

Fixes gogpu/ui#23

* fix: fallback attributes for unbound struct members, lint fixes

- Add fallback attribute generation for struct members without explicit
  bindings (fragment: [[position]]/[[user(locnN)]], vertex: [[attribute(N)]])
- Fix revive lint: globalIdBinding → globalIDBinding
- Add cyclop nolint for writeEntryPointInputStruct

Author: kolkov

CHANGELOG.md

@@ -5,6 +5,14 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.14.4] - 2026-03-01
+
+### Fixed
+
+#### MSL Backend
+- **Vertex `[[stage_in]]` for struct-typed arguments** — vertex shaders with struct-typed inputs (e.g., `fn vs_main(in: VertexInput)`) now correctly generate a synthesized `_Input` struct with `[[attribute(N)]]` members and `[[stage_in]]` parameter; previously only fragment stage was handled, causing undefined `in_` reference ([gogpu/ui#23](https://github.com/gogpu/ui/issues/23))
+- **`metal::discard_fragment()` namespace** — `discard_fragment()` now emits with required `metal::` namespace prefix; bare call was rejected by Metal shader compiler
+
 ## [0.14.3] - 2026-02-25
 
 ### Fixed