docs: add view-tag scanner architecture page

[brooksdevon001-star]

View-tag scanner architecture documentation

Adds architecture/view-tag-scanner.mdx — a dedicated architecture page explaining how the
1-byte view tag optimization works, written in response to repeated researcher questions about
scan performance.

What's in the page:

• Problem statement: naive scanning costs O(N) X25519 ECDH operations as the announcement log
  grows
• Derivation spec for both Stellar v2 (SHA-256("wraith:stellar:view-tag:v2:" || R || V)[0]) and
  EVM (keccak256(sharedSecret)[0])
• False-positive analysis: 1/256 rate, zero false-negatives, meaning no payments are ever
  missed
• Before/after cost structure diagram (view-tag-comparison.svg)
• Mermaid sequence diagram of the full scan loop with the tag-check branch
• Announcement batching: single-pass scanAnnouncements + batch_send sender-side grouping
• Benchmark methodology: full TypeScript harness using node:perf_hooks to reproduce the speedup
  measurement against a synthetic no-match set, with expected 200–280× range derived from first
  principles
• Domain separation table (v1 vs v2 prefixes with source line references to stealth.ts:8/9/99
  and scan.ts:12)
• Cross-chain comparison: Stellar, EVM, Solana, and CKB (which uses RPC-level pre-filtering
  instead)

Supporting changes:

• architecture/stellar-cryptography.mdx — forward link to the new page below the existing view
  tag diagram
• sdk/chains/stellar.mdx — note on scanAnnouncements pointing to the new page
• docs.json — page registered in the Architecture navigation group

Known gap: The acceptance criterion calls for a cross-reference from an SDK WebGPU guide, but
no such guide exists in the repo. That link should be added once the WebGPU guide is written.
closes #89

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[truthixify]

Merged. View-tag scanner architecture page is exactly what third-party indexers were asking for, nice work.