docs: correct blind-index claims (unkeyed SHA-256, not HMAC)

[p-diogo]

What

Several docs said word-level blind indices are HMAC outputs under a per-user secret and "cannot be inverted". In fact they're unkeyed SHA-256 of each token, its Porter stem, and entity names (rust/totalreclaw-core/src/blind.rs:22-49; entity trapdoors per docs/specs/totalreclaw/flows/02-write-path.md). So hashing a wordlist inverts them, and they're public on-chain. This makes the docs say so.

• docs/guides/beta-tester-guide-detailed.md:
  • write-path step 3 and the "Server-blind search" guarantee now describe unkeyed hashes and what that exposes
  • the "never sees query terms" line is qualified
  • step 2's content fingerprint is corrected too: it's HMAC-SHA256 under dedupKey, not plain SHA-256
• docs/specs/totalreclaw/architecture.md: the primitives table no longer says "cannot invert".
• docs/specs/totalreclaw/retrieval-v2.md, tiered-retrieval.md: diagram labels HMAC → SHA-256.
• docs/specs/totalreclaw/flows/02-write-path.md: adds a short "What blind indices reveal" note. It covers wordlist inversion, token order, on-chain visibility and query trapdoors reaching indexers, and notes that LSH buckets are per-user.

The fix itself (keyed word indices) is tracked separately.

Verification

Docs only. Claims checked against code:

• blind.rs (unkeyed sha256_hex(token) and sha256_hex("stem:"+stem), emitted in first-occurrence token order)
• mcp/src/subgraph/crypto.ts header (SHA-256 blind indices, HMAC-SHA256 content fingerprints)
• LSH bucket derivation from lshSeed per the write-path flow

docs/architecture.md already says SHA-256 and is left alone (it's also being edited in #674). The website's architecture page also says SHA-256, so it needs no change.

Checklist

• No recovery phrase, API key, or other secret appears anywhere in this diff or its logs.
• Docs touched by this change are updated (guides / feature-comparison / CLAUDE.md matrix where relevant).
• For behavior changes: E2E validated against staging (never production). (N/A, docs only)

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