Add paged GQA fused decode op, decouple Gemma4 from Qwen3.5, and improve cross-platform build docs

Added paged_gqa_decode_fused and related availability/input validation logic for paged KV decode.

Enhanced Qwen3DynamicCache page-table state handling and updated allocator behavior to align with fused-op capability.

Refactored Gemma4 input preparation, device, and scheduler/runtime integration to remove Qwen3.5 coupling.

Improved tools/build_gated_delta_fused_op.py for more robust platform-aware build behavior.

Updated README.md and README_zh.md to clarify environment requirements and setup/build instructions on both Windows and Linux.

Added/updated tests covering fused op behavior, scheduler/device safety, Gemma4 text/multimodal runtime, and build tooling.

Model/runtime/mm: src/anna/model/, src/anna/runtime/, src/anna/mm/*

Tooling/docs: tools/build_gated_delta_fused_op.py, README.md, README_zh.md

Tests: fused op, model shape/runtime, scheduler, multimodal, and build-script tests

新增 Paged GQA 融合解码算子，解耦 Gemma4 与 Qwen3.5，并完善跨平台构建文档

新增 paged_gqa_decode_fused 融合算子与可用性/输入校验逻辑，完善 paged KV 注意力解码路径。

扩展 Qwen3DynamicCache 与页表相关状态管理，更新分配器配置以适配新算子能力。

调整 Gemma4 相关输入准备、设备与调度逻辑，移除对 Qwen3.5 运行时耦合依赖。

更新 tools/build_gated_delta_fused_op.py，增强平台兼容性与构建流程可用性。

更新 README.md / README_zh.md，明确 Windows 与 Linux 的环境要求和安装/构建步骤。

新增/更新多项测试，覆盖融合算子、调度、设备安全、Gemma4 文本/多模态与构建脚本行为。

模型算子与运行时：src/anna/model/, src/anna/runtime/, src/anna/mm/*

构建工具与文档：tools/build_gated_delta_fused_op.py, README.md, README_zh.md

测试：tests/test_fused_op_xpu.py、tests/test_model_shapes.py、tests/test_gemma4_text_runtime.py 等