Add SentinelX project proposal and full SRS baseline docs

[Copilot]

This PR adds the two missing core planning artifacts requested for the project: a formal proposal document and a complete Software Requirements Specification (SRS). Together they establish project intent, scope boundaries, and implementation requirements in a reviewable baseline.

• Proposal document

  • Added docs/PROJECT_PROPOSAL.md.
  • Defines executive summary, problem statement, objectives, in/out scope, architecture, stakeholders, deliverables, risks, and success criteria.

• Complete SRS

  • Added docs/SRS.md.
  • Specifies external interfaces, functional requirements (FR-1 … FR-27), non-functional requirements (NFR-1 … NFR-13), data requirements, use cases, acceptance criteria, and validation strategy.

• Documentation baseline alignment

  • Captures SentinelX’s current architecture and operating model (packet capture → feature processing → ML inference → API/dashboard/DSL) in a requirements-driven format suitable for implementation and review.

docs/
├── PROJECT_PROPOSAL.md   # project intent, scope, milestones, risks, outcomes
└── SRS.md                # complete functional/non-functional requirements baseline