hrx: accept empty MoE expert views when matching graphs

[AaronStGeorge]

Overview

Qwen3 MoE microbatches that request no output logits produce empty expert views with nonzero slot offsets. HRX rejected these valid GGML views because their offsets exceed the zero-byte source extent, preventing graph matching during prefill and decode-like perplexity evaluation.

Skip byte-span bounds checks for empty tensors while preserving storage-root and alias-offset validation. Add a regression test covering all eight empty expert views, cache reuse, and rejection of a changed alias offset.

Validation

• Final source builds successfully; all 7 focused graph matching, cache UID, and zero-output tests pass.
• Earlier end-to-end validation on gfx1100: Qwen3-30B-A3B Q4_K_M perplexity with -c 512 --chunks 2 -b 512 -ub 1 completes with HRX PPL 6.7431 versus Vulkan 6.7573. The original code fails graph matching.
• Server boundary checks with -ub 256: original backend passes 256 prompt tokens and fails at 257; the repair completes 256, 257, and 513-token requests. These are execution checks, not generated-text quality checks.
• gfx1151 is covered by host-side graph construction, not GPU execution. The reported 262144-token all-GPU context could not be tested on the local 24 GiB GPU because its KV cache did not fit.

No HRX dependency update is required.

Requirements

• I have read and agree with the contributing guidelines.
• AI usage disclosure: YES. Codex assisted with investigation, implementation, tests, explanatory comments, and PR preparation under human review.