rtl: keep buffered KMU launches busy

[ForeverHYX]

Summary

• expose in-flight launch state from VX_kmu_arb;
• include buffered KMU requests in the device busy chain at every hierarchy
  level;
• add a Verilator regression for a backpressured registered fanout and wire it
  into the unit-test catalog.

Fixes #386

Root cause

VX_kmu clears running when the final CTA is accepted by the next fanout.
When that fanout has OUT_BUF=3, the CTA may still be held in a registered
output before a destination core accepts it. Downstream busy is also registered
through the hierarchy, so producer busy can fall before consumer busy becomes
visible. The resulting false-idle cycle lets a host treat an in-flight launch
as complete.

The fix reports pending while either side of VX_kmu_arb is valid and ORs
that state into the existing Vortex/cluster/socket busy aggregation. The core
graphics launch merge uses the same coverage. This changes only completion
visibility; it does not alter request arbitration, data, ready/valid
backpressure, or execution scheduling.

Regression test

The new hw/unittest/kmu_arb test instantiates a 1-to-2 arbiter with
OUT_BUF=3, transfers one launch into its registered output, then blocks the
selected destination for three cycles. It checks that:

1. a stalled input asserts pending;
2. pending remains high when source valid drops and output valid owns the
   request;
3. backpressure does not pulse or clear pending;
4. the output handshake cycle remains covered;
5. pending clears after the fanout drains.

The test is registered as unittest:hw-kmu-arb for both XLEN configurations.

Validation

All commands used an out-of-tree build configured with
--tooldir=$HOME/tools.

• Sensitivity check with output occupancy intentionally removed:

  buffered handoff: in_valid=0 out_valid=0x1 pending=0
  FAIL: pending must bridge the source-to-buffer ownership transfer
  

• Restored implementation, XLEN=32 catalog case:

  1 passed, 547 deselected
  

• Restored implementation, XLEN=64 catalog case:

  1 passed, 483 deselected
  

• Existing adjacent unit tests:

  make -C hw/unittest/kmu run             # pass
  make -C hw/unittest/cta_dispatcher run  # pass
  

• Two-core RTL system checks:

  CONFIGS='-DVX_CFG_SOCKET_SIZE=2' ./ci/blackbox.sh \
      --driver=rtlsim --app=basic --cores=2 --args='-n4'
  # Test PASSED; 74 instructions, 540 cycles
  
  ./ci/blackbox.sh --driver=rtlsim --app=demo --cores=2
  # PASSED; 4,888 instructions, 11,525 cycles
  

• Hardware synthesis smoke:

  make -C hw/syn/yosys
  # Yosys mapping and OpenSTA completed successfully
  

• git diff --check upstream/master: pass.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.