Fix: disambiguate paged attention event casts

[puddingfjz]

Summary

Fix A5 onboard compilation failures in paged-attention AIC kernels by replacing
ambiguous C-style event_t casts with explicit global event_t casts.

A5's onboard toolchain sees both CANN's global event_t and PTO-ISA's
pto::event_t, so a C-style event_t cast can fail name lookup with:

reference to 'event_t' is ambiguous

Scope

• Update paged-attention AIC kernels in A5 and matching A2/A3 example/ST files.
• Keep the cast target explicitly global so it selects the CANN intrinsic event type.
• No Python callable or golden tolerance changes are included.

Validation

• rg "\(event_t\)" examples tests src python has no remaining matches.
• clang-format --dry-run --Werror passed on all touched files.
• git diff --cached --check passed before commit.

This was exposed by st-onboard-a5 in PR #839, specifically:

examples/a5/tensormap_and_ringbuffer/paged_attention_unroll_manual_scope/test_paged_attention_unroll.py::TestPagedAttentionUnrollManualScope::test_run

[gemini-code-assist]

Code Review

This pull request replaces C-style casts with static_cast<::event_t> in several aic_pv_matmul.cpp files across examples and tests. These changes affect the wait_flag and set_flag calls within the pv_matmul_n_impl function, promoting better C++ practices and type safety. I have no feedback to provide as there were no review comments to evaluate.