Implement AXI-Stream UVM agent

[allRisc]

Build out the previously-empty axi_stream_agent submodule into a full UVM agent, mirroring the structure and design decisions of the sibling APB and AHB agents.

Agent

• Two operating modes selected via axi_st_agent_config.agent_mode: AXI_ST_TRANSMITTER_AGENT drives TVALID and the transfer payload from a sequence; AXI_ST_RECEIVER_AGENT drives TREADY, applying reactive backpressure. Mirrors the APB requester/completer split.
• One transaction models a single stream transfer (beat), carrying data/strb/keep/last/id/dest/user plus per-transfer tvalid_delay (idle insertion) and tready_delay (backpressure) knobs.
• Optional sidebands TID/TDEST/TUSER default to width 0 and are guarded by (W==0)?1:W localparams so zero-width vectors never occur (same technique the AHB agent uses for HMASTER_WIDTH).

Components

• axi_st_vip_if interface with drv_tx_cb / drv_rx_cb / mon_cb clocking blocks and matching modports, driven off aclk / areset_n.
• Driver implements complementary transmitter and receiver run phases over the handshake, honouring the idle/backpressure delays.
• Monitor emits one transaction per completed TVALID && TREADY handshake and forwards offered transfers to reactive receiver sequences via req_ap.
• Sequencer, coverage collector, and agent wire-up follow the APB layout.
• Reactive receiver sequences: base reactive, no-backpressure, and randomized backpressure (parallels APB's reactive/no-wait/wait sequences).

Parameter checking

• axi_st_agent.check_params enforces TDATA width as a positive multiple of 8 and warns on TID > 8 / TDEST > 4 per the AXI-Stream recommendations.

Example / tooling

• Self-contained example testbench (env, transmitter sequence, reactive receiver, top module) plus filelists, proj.toml, Natural Docs config, and the GitHub Pages workflow, matching the other agents.

Naming / license

• Repo-level identifiers keep the full axi_stream form (package axi_stream_agent_pkg, axi_stream_agent.f); all internal types, classes, macros, and the interface use the axi_st_ / AXI_ST_ abbreviation.
• All source files carry the MIT header matching this repo's LICENSE.

Correctness notes (deliberate improvements over the APB reference)

• The strb_keep_c constraint (TSTRB may only assert where TKEEP asserts) is guarded to transmitter-generated stimulus so a reactive randomize() cannot fail on monitor-sampled values.
• The coverage covergroup is constructed in the collector's constructor.
• The monitor's request latch clears whenever TVALID is idle, so a withdrawn offer cannot deadlock reactive receiver sequences.