Update synthetic ID generation: new components, random suffix, and IPv6 normalization

[aram356]

Summary

This PR updates the synthetic ID generation system with three key improvements:

1. Updated Template Components (Closes #207)

Components kept:

• client_ip - Client IP address (now normalized for IPv6)
• user_agent - Browser user agent string
• accept_language - Accept-Language header

Components added:

• accept_encoding - Accept-Encoding header
• random_uuid - Random UUID for additional entropy

Components removed:

• first_party_id
• publisher_domain
• auth_user_id

New default template:

{{ client_ip }}:{{ user_agent }}:{{ accept_language }}:{{ accept_encoding }}

2. Random Suffix for Uniqueness

Appends a random 6-character alphanumeric suffix (a-z, A-Z, 0-9) to the HMAC hash, separated by a period.

Final ID format: <64-char-hmac>.<6-char-random-suffix>

Example: a1748067b3908f2c9e0f6ea30a341328ba4b84de45448b13d1007030df14a98e.xK9f2B

3. IPv6 Address Normalization (Closes #209)

Some devices rotate their IPv6 suffix frequently (Privacy Extensions), which would break the SyntheticID's persistence. This change normalizes IPv6 addresses to the /64 prefix (industry standard for stability).

• IPv4 addresses are passed through unchanged
• IPv6 addresses are masked to /64 prefix (first 4 segments kept, rest zeroed)
• Two devices with the same /64 prefix but different interface identifiers will generate the same base ID

Files Changed

• crates/common/src/synthetic.rs - Main synthetic ID generation logic
• trusted-server.toml - Default configuration and template documentation

Test plan

• All existing tests pass (306 tests)
• cargo fmt - no formatting issues
• cargo clippy - no warnings
• New tests for IPv6 normalization
• New tests for random suffix format validation
• Verify synthetic ID generation works correctly in staging environment