v0.3.5: Encrypted receipt & comment slots, message delivery indicators

Encrypted slots in BlobHeader:
- Private posts get noise-prefilled receipt slots (64B, 1 per participant)
  and comment slots (256B, ceil(participants/3), expandable)
- Slot key derived from post CEK via BLAKE3 — only participants can read
- CDN relays propagate opaque encrypted bytes without decryption
- 3 new BlobHeaderDiffOps: WriteReceiptSlot, WriteCommentSlot, AddCommentSlots

Receipt system:
- States: empty(0), delivered(1), seen(2), reacted(3)
- Slot index = position in sorted participant NodeId list
- Author can pre-feed emoji reaction at creation time
- 6 new crypto tests for slot encrypt/decrypt/derivation

Node methods:
- write_receipt_slot, write_comment_slot with upstream+downstream propagation
- read_receipt_slots, read_comment_slots with CEK-based decryption
- get_post_cek_and_participants helper for both Encrypted and GroupEncrypted

IPC: write_message_receipt, write_message_comment, get_message_receipts,
     get_message_comments

Frontend:
- DM chat bubbles show delivery indicators (check → double → blue → emoji)
- Opening conversation auto-marks incoming messages as seen
- React button on messages with emoji prompt

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

website/download.html

@@ -78,6 +78,9 @@
                     <li><strong>Crypto refactoring</strong> &mdash; Extracted reusable primitives: <code>encrypt_bytes_with_cek</code>, <code>decrypt_bytes_with_cek</code>, <code>unwrap_cek_for_recipient</code>, <code>unwrap_group_cek</code>. Foundation for encrypted blob storage and future chunk-level encryption.</li>
                     <li><strong>Intent-based post filtering</strong> &mdash; Feed, My Posts, and Messages now filter on the author's original visibility intent (<code>intentKind</code>) rather than encryption state. Direct messages are identified by intent, not by being &ldquo;encrypted-for-me.&rdquo; Backward-compatible with pre-intent posts.</li>
                     <li><strong>Blob decryption on retrieval</strong> &mdash; New <code>get_blob_for_post</code> API decrypts private blobs in context of their post&rsquo;s visibility. Public blobs pass through unchanged.</li>
+                    <li><strong>Encrypted receipt slots</strong> &mdash; Private messages get encrypted receipt and comment slots in their BlobHeader. Pre-filled with random noise so slot writes are indistinguishable from creation. Receipt states: delivered, seen, reacted. Only participants with the CEK can read slots; relay nodes propagate opaque bytes.</li>
+                    <li><strong>Message receipts &amp; reactions</strong> &mdash; DM conversations show delivery indicators (checkmark &rarr; double checkmark &rarr; emoji). Opening a conversation marks messages as seen. React to messages with emoji.</li>
+                    <li><strong>Private comment slots</strong> &mdash; Encrypted comment capacity in private post headers (ceil(participants/3) slots, expandable). Participants can write short comments that propagate via CDN without revealing content to relays.</li>
                     <li><strong>Download filename sanitization</strong> &mdash; Prevents path traversal in downloaded file names.</li>
                 </ul>