chore(release): prepare Core 1.6.0

[QuickMythril]

Bumps Qortium Core to 1.6.0 for the next preview prerelease.

What's in 1.6.0 (since v1.5.1)

• Avatars — immutable QDN avatars for accounts and groups (feat(avatars): authorize immutable QDN group and account avatars #152)
• AT engine v3 — version-3 ATs gated at height 70000 (feat(at): gate version-3 AT deployment behind an activation height #155), persistent maps + cross-AT reads (feat(at): add persistent maps and cross-AT reads #150), chain-query opcodes for trust/balances/code-hashes (feat(at): chain-query opcodes — trust status, account balance, code-hash check #153)
• AT consensus hardening — checked arithmetic, asset sweep on finish, hashing-step cost, payout solvency, local-height gates (fix(at): consensus-safety hardening — checked arithmetic, asset sweep, hashing cost, local-height gates #154, fix(at): enforce payout solvency and pin QortiumDev runtime #149)
• QDN/gateway — read-only QDN actions + browsing in gateway mode (feat(gateway): serve read-only QDN actions in gateway mode #145, feat(qdn): theme the loading splash and add gateway QDN browsing #146), QDN apps inherit host uiStyle (feat(qdn): give QDN apps the uiStyle their host is using #147), raw-download stored-XSS sandbox (fix(api): sandbox raw QDN downloads against stored XSS #157)
• API/network robustness — restore tx building for 20 tx types (fix(api): restore transaction building for twenty transaction types #161), clear 400 on malformed bodies (fix(api): answer malformed request bodies with a clear 400 instead of a blank 500 #162), concurrent-safe resource lists (fix(list): make resource lists safe to read while they are being written #160), reject negative declared lengths (fix(network): reject negative declared lengths instead of killing the I/O thread #158)

Chain config

previewchain.json adds 10 activation heights, all at 70000 (AT v3 + avatarTransactionsHeight). Chain is at ~63.8k so nothing has activated yet. This is a coordinated peering flag-day — all preview seeds must roll to 1.6.0 together.

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