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>

frontend/style.css

@@ -286,8 +286,16 @@ header h1 { font-size: 1.4rem; color: #7fdbca; }
 .chat-mine { align-self: flex-end; background: #0f3460; color: #e0e0e0; border-bottom-right-radius: 4px; }
 .chat-theirs { align-self: flex-start; background: #1e2040; color: #dde; border-bottom-left-radius: 4px; }
 .chat-text { white-space: pre-wrap; }
-.chat-time { font-size: 0.6rem; color: #778; margin-top: 0.2rem; text-align: right; }
+.chat-meta { display: flex; align-items: center; gap: 0.3rem; justify-content: flex-end; margin-top: 0.15rem; }
+.chat-theirs .chat-meta { justify-content: flex-start; }
+.chat-time { font-size: 0.6rem; color: #778; text-align: right; }
 .chat-theirs .chat-time { text-align: left; }
+.receipt-indicator { display: inline-block; min-width: 1em; }
+.receipt-check { font-size: 0.65rem; color: #778; letter-spacing: -0.15em; }
+.receipt-check.seen { color: #7fdbca; }
+.receipt-check.reacted { color: #f0a; letter-spacing: 0; }
+.slot-react-btn { background: none; border: 1px solid #444; color: #aab; border-radius: 10px; font-size: 0.6rem; padding: 0 0.3rem; cursor: pointer; line-height: 1.4; opacity: 0; transition: opacity 0.15s; }
+.chat-bubble:hover .slot-react-btn { opacity: 1; }
 
 .conv-reply { display: flex; gap: 0.4rem; align-items: flex-end; margin-top: 0.5rem; padding-top: 0.5rem; border-top: 1px solid #2a2a40; }
 .conv-reply-input { flex: 1; padding: 0.4rem; background: #1a1a2e; color: #e0e0e0; border: 1px solid #333; border-radius: 4px; resize: none; font-family: inherit; font-size: 0.85rem; min-height: 36px; line-height: 1.4; transition: border-color 0.15s; }