Remove Loki tx_extra support and harden vector deserializer

[MoneroOcean]

Motivation

• Address a high-risk DoS vector where newly-registered Loki tx_extra types exposed attacker-controlled std::vector counts that could trigger large reserve allocations during parsing.
• Project decision: Loki support is no longer required, so eliminating Loki-specific tx_extra types both removes the attack surface and simplifies maintenance.

Description

• Removed Loki service-node and key-image tx_extra constants, types, serializers, and variant registrations in src/cryptonote_basic/tx_extra.h so those tags are no longer recognized by the parser.
• Removed the BLOB_TYPE_CRYPTONOTE_LOKI enum entry from src/cryptonote_config.h and updated code paths that previously referenced Loki.
• Renamed/re-scoped the previous Loki/XTNC naming to XTNC-specific symbols (xtnc_version / xtnc_type) and limited XTNC/transaction-type branches to BLOB_TYPE_CRYPTONOTE_XTNC in src/cryptonote_basic/cryptonote_basic.h and src/main.cc.
• Mitigated the vector-allocation amplification by removing the pre-parse v.reserve(cnt) call from src/serialization/vector.h, preventing attacker-controlled counts from causing large premature allocations.

Testing

• Ran rg searches for Loki/service-node/key-image symbols and confirmed no remaining matches (search returned no hits). (succeeded)
• Ran git diff --check to validate whitespace/diff issues. (succeeded)
• Attempted npm test but it was blocked by registry access errors (403 Forbidden fetching bech32), so full JS test execution could not complete. (blocked)
• Attempted a local C++ syntax-only compile but it failed due to missing Boost headers in the environment, so a full native build could not be validated here. (blocked)

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Codex Task