feat: paged storage, O(1) lists, bucketed user intents, intent pruning

[villadel]

Resolves #153, #371, #373, #374.

Issue #153 — Bucketed UserIntents

• Replace single UserIntents(Address) Vec with fixed-size buckets of 64 intent IDs: UserIntents(Address, u32) + UserIntentCount(Address).
• list_intents_by_user(user, start, limit) is now paginated; pruned slots (None) are skipped. Backward-compatible list_intents_by_user_legacy kept.
• submit_intent writes via user_intents_append(); bump_user_intents_ttl() keeps the active bucket live.

Issue #373 — Paged persistent SolverList + AllowedDstTokenList

• Move both lists from instance storage (O(n) per-call deserialisation) to paged persistent storage: SolverListPage(u32) / SolverListCount / SolverListIdx(Address) and matching DstTokenList* variants.
• add_to_solver_list / remove_from_solver_list and add_to_dst_token_list / remove_from_dst_token_list are now O(1) via append + index map (swap-remove on delete).
• list_solvers and list_allowed_dst_tokens updated to walk pages.
• Ordering is not insertion order after any swap-remove; documented.

Issue #374 — O(1) SolverIntents via index map

• Add SolverIntentIdx(Address, BytesN<32>) -> u32 alongside SolverIntents.
• solver_intents_add: O(1) push + index write; duplicate-add is a no-op.
• solver_intents_remove: O(1) swap-remove + index update.
• get_solver_intents(solver, start, limit) is now paginated.
• Invariant: len(SolverIntents(solver)) == solver_record.active_intents.

Issue #371 — Permissionless close_intent + tombstone

• Add terminal_at: Option field to IntentRecord; set on every terminal transition (Filled, Cancelled, Expired, Resolved).
• close_intent(intent_id): permissionless; callable by anyone after terminal_at + retention_secs <= now (default 30 days). Deletes: Intent, IntentFillHistory, ExtensionGranted, BestBid, BackstopClaimed, UserIntents bucket slot, SolverIntents entry. Leaves IntentTombstone so IntentAlreadyExists still fires on id reuse.
• set_intent_retention(secs) / get_intent_retention() for admin config.
• New errors: IntentNotTerminal (36), RetentionPeriodNotElapsed (37).
• Updated docs/145-ttl-bump-frequency-review.md §5.

Housekeeping

• Deduplicate accumulated duplicate constant definitions (leftover merges).
• Update .gitignore: add *.snap, *.snap.new, coverage artifacts, criterion output, and other unnecessary files.

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
closes #371
closes #372
closes #373
closes #374

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