Document StreamAccessibleFifo and StreamShiftChain

[Copilot]

Two stream utilities introduced in SpinalHDL/SpinalHDL#1418 lacked documentation. This PR adds reference docs for both in stream.rst under the Utils section.

StreamShiftChain

Shift-register pipeline chain (renamed from StreamHistory). Creates length M2S pipeline stages with all intermediate states observable via io.states.

• Parameter and IO port tables
• clear signal documented (flushes pipeline)
• Usage examples for both direct instantiation and companion object

StreamAccessibleFifo

FIFO where all stored elements are simultaneously visible via io.states. Pushes route to the first empty slot; elements shift toward output on pop.

• Parameter and IO port tables
• Usage examples for both direct instantiation and companion object

// StreamShiftChain
val chain = StreamShiftChain(inputStream, outputStream, length = 4)
val stageN = chain.io.states(n)   // observe any stage
chain.io.clear := someCondition   // flush

// StreamAccessibleFifo
val fifo = StreamAccessibleFifo(inputStream, outputStream, length = 4)
val slot0 = fifo.io.states(0)     // observe any slot

Original prompt

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This section details on the original issue you should resolve

<issue_title>Document StreamAccessibleFifo and StreamShiftChain</issue_title>
<issue_description>Do not merge until SpinalHDL/SpinalHDL#1418 has not been merged</issue_description>

<agent_instructions>check the code in SpinalHDL/SpinalHDL and write the document here.</agent_instructions>

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

• Fixes Document StreamAccessibleFifo and StreamShiftChain #256

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