Fixes hw-native-sys#1483

`tests/st/a2a3/host_build_graph/bgemm` declared its `C` argument `D.OUT`,
but the task graph read-modify-writes it: `p_add.add_inout(c_view)` feeds
`kernel_tile_add`, whose first `k` iteration `TLOAD`s `C` before any task
has written it.

The runtime stages only IN/INOUT tensors host->device
(`runtime_maker.cpp`, "pure write-only OUTPUT buffers are never read by
the kernel"), and since hw-native-sys#1365 it no longer zero-fills OUT buffers either.
So `C` accumulated onto whatever the device allocation happened to hold.
On sim that allocation is a plain `malloc` block on a process-lifetime
worker: fresh zeros when the case runs alone, a previous case's bytes in
a full-suite run — hence a golden mismatch of 1e25-1e36 that reproduced
only under the full suite. Same class as hw-native-sys#1449.

Declare `C` INOUT, matching the two sibling bgemm variants
(`examples/a5/tensormap_and_ringbuffer/bgemm`,
`examples/a2a3/tensormap_and_ringbuffer/benchmark_bgemm`), which already
tag their accumulator that way.

Seed `C` with a non-zero base (0.25) so the staging is load-bearing and
the case is deterministic: golden is now `C_init + A @ B`, which a
zeroed or unstaged device buffer cannot match. Without the signature fix
this fails every run instead of one in two or three — verified, it
reproduces the reported 1e35 magnitude on the first attempt.

Drop the `pytest.mark.skip` that quarantined the case: the underlying
defect is fixed, so the case runs in CI again.

Verified: bgemm passes 3/3 in isolation on a2a3sim and onboard on a2a3;
the full `pytest examples tests/st --platform a2a3sim` suite is green
3/3 runs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

)

Fixes #1483

`tests/st/a2a3/host_build_graph/bgemm` declared its `C` argument `D.OUT`,
but the task graph read-modify-writes it: `p_add.add_inout(c_view)` feeds
`kernel_tile_add`, whose first `k` iteration `TLOAD`s `C` before any task
has written it.

The runtime stages only IN/INOUT tensors host->device
(`runtime_maker.cpp`, "pure write-only OUTPUT buffers are never read by
the kernel"), and since #1365 it no longer zero-fills OUT buffers either.
So `C` accumulated onto whatever the device allocation happened to hold.
On sim that allocation is a plain `malloc` block on a process-lifetime
worker: fresh zeros when the case runs alone, a previous case's bytes in
a full-suite run — hence a golden mismatch of 1e25-1e36 that reproduced
only under the full suite. Same class as #1449.

Declare `C` INOUT, matching the two sibling bgemm variants
(`examples/a5/tensormap_and_ringbuffer/bgemm`,
`examples/a2a3/tensormap_and_ringbuffer/benchmark_bgemm`), which already
tag their accumulator that way.

Seed `C` with a non-zero base (0.25) so the staging is load-bearing and
the case is deterministic: golden is now `C_init + A @ B`, which a
zeroed or unstaged device buffer cannot match. Without the signature fix
this fails every run instead of one in two or three — verified, it
reproduces the reported 1e35 magnitude on the first attempt.

Drop the `pytest.mark.skip` that quarantined the case: the underlying
defect is fixed, so the case runs in CI again.

Verified: bgemm passes 3/3 in isolation on a2a3sim and onboard on a2a3;
the full `pytest examples tests/st --platform a2a3sim` suite is green
3/3 runs.