Document simulation API for watching uninitialized input signals

[Copilot]

SpinalHDL 1555 introduced an API to detect undriven DUT input signals during simulation, but it was undocumented. Adds a new section to Simulation/signal.rst covering all four new functions.

Changes

• source/SpinalHDL/Simulation/signal.rst — New "Watching uninitialized input signals" section with an API reference table and usage example:

  • addInputsAssignmentWatch(dut) — mark all module inputs as watched
  • addWatchedSignals(signals) — mark an explicit Seq[BaseType] as watched
  • checkInputsAssignmentAfterReset(cd, stopOnFail) — after each clock sampling, emit warnings (or call simFailure) for any watched input that has never been driven
  • checkWatchedSignalAssigned() — returns names of currently undriven watched signals

• .gitignore — Added build/ to exclude Sphinx HTML output from the repo.

Example

SimConfig.compile(new MyTopLevel).doSim { dut =>
  addInputsAssignmentWatch(dut)          // watch all inputs
  dut.clockDomain.forkStimulus(10)
  checkInputsAssignmentAfterReset(dut.clockDomain, stopOnFail = false)

  // forgetting to drive dut.io.enable here triggers a warning each cycle
  dut.clockDomain.waitSampling(100)
  simSuccess()
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Document watch on uninitialized input signals.</issue_title>
<issue_description>Do not merge until SpinalHDL/SpinalHDL#1555 has not been merged</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Document watch on uninitialized input signals. #266

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