docs(security): STRIDE threat model reference page (#85)

[Charity5654]

Add reference/threat-model.mdx with per-contract STRIDE threat mapping for all 14 Wraith Protocol contracts and scripts across EVM, Stellar, Solana, and CKB. Includes 8 cross-cutting residual risks, an audit history table, and bidirectional cross-links with the SAC compatibility audit page.

EVM (5 contracts):

• ERC5564Announcer: E-01 to E-06
• ERC6538Registry: E-07 to E-13
• WraithSender: E-14 to E-19
• WraithNames: E-20 to E-27
• WraithWithdrawer: E-28 to E-32

Stellar (4 contracts):

• stealth-announcer: S-01 to S-04
• stealth-registry: S-05 to S-10
• stealth-sender: S-11 to S-18
• wraith-names: S-19 to S-25

Solana (3 programs):

• wraith-announcer: L-01 to L-04
• wraith-sender: L-05 to L-09
• wraith-names: L-10 to L-16

CKB (2 scripts):

• wraith-stealth-lock: C-01 to C-10
• wraith-names-type: C-11 to C-19

Residual risks: R-01 (ephemeral key reuse) through R-08 (upgrade authority).

• Add reference/threat-model to Contracts nav group in docs.json
• Add See Also section to 2026-06-sac-compatibility.md linking back to the threat model (R-05 covers SAC flag hazards)

Closes #85

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[truthixify]

Merged, thanks. Threat model as a public reference gives partners the artifact they have been asking for.