Create trust_security_model.md

[Clawue884]

This pull request introduces a new documentation file: trust_security_model.md.
The document provides a unified, end-to-end explanation of trust, security, and verification across the Pi ecosystem, including:
Pi Browser & Frontend JS SDK
Pi Platform authentication and payments
App backend approval/completion flows
Pi Blockchain ledger finality
PiNet metadata and token minting
It consolidates concepts that are currently documented separately (auth, payments, tokens, PiNet) into a single architectural trust model, including:
Authentication and identity guarantees
Server-verified payment flows
Threat model and mitigations
Rationale for Pi’s non-smart-contract-first design
This change:
Adds one documentation file only
Introduces no API or SDK changes
Is intended to improve clarity for developers, auditors, and reviewers

[Clawue884]

This document is intended as a conceptual overview of the
Pi Platform trust architecture.

It consolidates authentication, payments, token minting,
and PiNet metadata into a single model to help:

• developers understand trust boundaries
• auditors analyze payment security
• ecosystem builders reason about token provenance

If needed, the document can be split into smaller sections
or integrated into existing docs.