feat: opt-in backstop pool for slashed-intent user compensation

[Keengfk]

• Add DataKey::BackstopPool (instance) and BackstopClaimed(BytesN<32>) (persistent) storage keys
• Add Error::BackstopPoolEmpty=35 and BackstopAlreadyClaimed=36
• Add MAX_BACKSTOP_BPS (5 000) and MAX_BACKSTOP_CLAIM_BPS (100) constants
• Add missing constants: DEFAULT_MIN_BOND, DEFAULT_FILL_WINDOW, DEFAULT_INTENT_EXPIRY, DEFAULT_PROTOCOL_FEE_BPS, MAX_PROTOCOL_FEE_BPS, MIN_FILL_WINDOW_SECS, MIN_INTENT_EXPIRY_SECS, MIN_BOND_FLOOR, SLASH_COOLDOWN, CANCEL_COOLDOWN, MAX_BATCH_SIZE, MAX_EXTENSION_DURATION
• Add missing DataKey variants: Config, PendingAdmin, AllowedDstTokenList, PendingDstTokenAdd, PendingDstTokenRemove, MinBondMultiplier, UserIntents, CancelCooldown, ExtensionGranted
• Fix Error enum duplicate discriminants; add TimelockNotElapsed=26, NoPendingAdminTransfer=27, NoPendingDstTokenChange=28, InvalidSrcToken=29, AmountTooLarge=30, InvalidConfig=31, CancelCooldownNotExpired=32, SrcChainNotAllowed=33, RescueProtectedToken=34
• Add backstop_bps: i128 field to ProtocolConfig (default 0 = dormant)
• Update set_config to accept and validate backstop_bps (0..=MAX_BACKSTOP_BPS)
• Modify slash_solver: divert backstop_bps share into BackstopPool before transferring remainder to fee_recipient (CEI discipline preserved); event payload extended to (intent_id, slash_amount, backstop_amount)
• Add claim_backstop_compensation entrypoint: user calls once per intent, payout capped at MAX_BACKSTOP_CLAIM_BPS (1%) of pool, min 1 stroop; BackstopClaimed flag prevents double-claims across re-slash cycles
• Add get_backstop_pool view function
• Add get_pending_admin view function (was missing)
• Fix fill_intent: remove duplicate premature token transfers (triple-send bug)
• Fix existing tests: pass &caller to pause() (was zero-arg in 3 tests)
• Add backstop pool test suite: accumulation, successful claim, double-claim rejection, empty pool / bps=0 rejection, config validation tests

Summary

Related issue

Type of change

• Bug fix
• New feature
• Refactor
• Documentation
• CI / tooling

Component

• Contract (vortex-contract)
• Backend (vortex-backend)
• Frontend (vortex-frontend)

Checklist

• My code follows the project's style and conventions
• I ran lint / type-check / build locally and they pass
• I added or updated tests where appropriate
• I updated documentation where appropriate
• My commits follow Conventional Commits

Screenshots / notes

closes #280

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