Add contract stream templates

[itzkingaddie]

Overview

Adds reusable contract stream templates so senders can save stream configuration and create funded streams from those saved templates.

Related Issue

Closes #684

Changes

• Added StreamTemplate and TemplateCreated contract types in contracts/src/templates.rs.
• Added persistent storage for template IDs, template records, and per-sender template indexes.
• Added template APIs:
  • create_template
  • get_template
  • get_templates_by_sender
  • create_stream_from_template
• Added vesting_type to stream records and stream-created events so template-created streams retain the saved config.
• Reused the existing stream validation, token allowlist, escrow transfer, stream ID allocation, storage, and event path for template-created streams.
• Isolated the legacy escrow contract in its own module and declared errors.rs, fixing pre-existing library compile blockers from duplicate Soroban macro symbols and the missing module declaration.
• Merged latest main and preserved upstream claim throttling, native token admin, DAO, benchmark, and snapshot changes while resolving the template-stream conflict.

Verification Results

• cargo check --manifest-path contracts\Cargo.toml --lib passes.
• cargo test --manifest-path contracts\Cargo.toml --test templates passes: 2 passed, 0 failed.
• cargo test --manifest-path contracts\Cargo.toml passes: 132 unit tests, 1 benchmark test, 2 template tests, and doc tests.

Acceptance Criteria

Criteria	Status
Template creation stores config on-chain	Done
Stream from template has identical config	Done
Max 10 templates per sender	Done

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