BE — Add explainers for all remaining Stellar operation types

[Tinna23]

Description

The backend currently only explains payment operations. All other operation types fall into Operation::Other and return no explanation. Stellar has ~20 operation types — this issue implements explainers for the most important remaining ones, following the exact same pattern as the existing payment.rs explainer.

Operation Types to Implement

Each operation needs:

• A struct in src/models/operation.rs
• A From<HorizonOperation> mapping
• An explainer in src/explain/operation/
• Unit tests

Required operations (in priority order):

Operation	Why it matters
create_account	Most common after payment — activates new wallets
change_trust	How accounts opt in to hold assets (USDC etc.)
set_options	Configures signers, flags, home domain, thresholds
path_payment_strict_send	Cross-asset payment — sends one asset, receiver gets another
path_payment_strict_receive	Cross-asset payment — receiver specifies exact amount
manage_sell_offer	DEX order placement / update / cancel
manage_buy_offer	DEX buy order
create_passive_sell_offer	DEX passive order (doesn't consume existing offers)
clawback	Regulated asset recovery by issuer
clawback_claimable_balance	Clawback on claimable balances
account_merge	Merges account into another, transfers all XLM

Requirements

• Follow the exact pattern established in src/explain/operation/payment.rs
• Each explainer must be pure, deterministic, and side-effect free
• Each new variant must be added to the Operation enum in src/models/operation.rs
• Each must have a From<HorizonOperation> mapping
• Register each new module in src/explain/operation/mod.rs
• Plain-English explanations — no jargon
• Example output for create_account: "GABC… funded and activated a new Stellar account GDEST… with a starting balance of 1.5 XLM"
• Example output for change_trust (add): "GABC… opted in to hold up to 10,000 USDC issued by GBBB…"
• Example output for change_trust (remove): "GABC… removed their trust line for USDC"
• Example output for account_merge: "GABC… merged their account into GDEST…, transferring all remaining XLM"
• DEX offers: offer_id = 0 = new offer, offer_id > 0 = update, amount = 0 = cancel

Key Files

• packages/core/src/models/operation.rs
• packages/core/src/explain/operation/ (create new files here)
• packages/core/src/explain/operation/mod.rs
• packages/core/src/services/horizon.rs (add any missing HorizonOperation fields)

Acceptance Criteria

• All 11 operation types listed above have a working explainer
• Each explainer has unit tests covering standard and edge cases
• Fetching a create_account transaction returns a human-readable explanation
• Fetching a change_trust transaction correctly identifies add vs remove
• DEX offers correctly identify new offer vs update vs cancel
• cargo test passes with no regressions
• No operation type returns an empty or missing explanation

Stage: S3 — Operation Coverage