[ESI] ChannelMMIO: decrease MMIO space per client

[teqdruid]

Reduces the amount of space assigned to each MMIO client to 1k (which fits 128 64-bit registers). Deals with manifest reads correctly.

[Copilot]

Pull Request Overview

This PR reduces the MMIO space allocated per client from 1MB to 1KB (128 64-bit registers) and improves manifest ROM handling. The changes optimize memory usage while ensuring proper address decoding for both regular clients and manifest reads.

• Updated MMIO space constants from 0x100000 to 0x400 (1KB per client)
• Enhanced address decoding logic to handle manifest reads separately from regular client access
• Removed TODO comment about insufficient address space for large compressed manifests

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