feat(indexer): keyset pagination, holders, order book (api-server v2)

[nullPointerEnjoyer]

Implements the api-server v2 changes from mintlayer-core PR #2130 (commit 2684078c546, api-server 1.4.1), mirroring the existing SDK structure (one client method per endpoint, request-option pattern, typed responses, typed errors). No breaking changes to existing method signatures — the one behavioral fix is Transaction's pending-tx fields becoming Option so mempool entries decode.

New endpoints & response types

• GET /statistics/coin/holders, GET /statistics/token/{id}/holders: address-balance holders listing (Holder { address, amount }, amounts rendered with the coin's/token's decimals). Unknown token → Error::TokenNotFound.
• GET /order/pair/{pair}/book: aggregated order book per side (OrderBook/OrderBookLevel/OrderBookPrice, exact numer/denom price plus floored-toward-zero decimal). Side-specific cursors (ask cursor on a bid walk → Error::InvalidCursor). Truncation invariant encoded in the type: truncated: true always arrives with next_cursor: null, documented, and the pager stops on it. Coin tickers match case-insensitively; token ids are preserved exactly (bech32 is case-sensitive).
• Statistics docs now state the four supply counters (circulating_supply, preminted, burned, staked) are always present; unwritten counters are zero.

Cursor pagination

• Envelope type Page<T> { items, next_cursor } and a reusable Pager<T> walker (*_pager constructors for pools, the global transaction listing, both holders listings, and the order book): transparently follows next_cursor, stops on null, keeps its position across failed fetches (retriable), typed errors surface as Some(Err).
• Page sizes clamped client-side to 1..=100 (MAX_NUM_ITEMS); server-side items=0/items>100 rejection mapped to Error::InvalidNumItems.
• list_transactions_with_offset_mode (OffsetMode::Legacy/Absolute); combining cursor + offset_mode → Error::BadRequest (documented). Pools: cursors only with the default by_height sort (documented; other sorts keep the offset-based list_pools).
• Cursor-overrides-offset semantics documented (not "fixed" client-side); page stability only guaranteed once the scanner is caught up (documented per endpoint).

Typed errors

Error::InvalidCursor, InvalidNumItems, BadRequest, TokenNotFound, mapped from exact {"error": ...} server bodies; anything else keeps the existing Error::Http fallback.

Tests & docs

• 14 new wire tests (httpmock fixtures): full cursor walks (holders, pools, global transactions incl. pending null block_id), both book sides, truncated book stops the pager, last page, invalid-cursor/invalid-num-items/token-not-found, page-size clamping, retry-resume after a failed fetch, start_from rewind.
• docs/indexer.md + README: per-endpoint cursor rules, error table with exact wire messages, runnable snippets; new examples/indexer-pagination.rs (holders, both-side book, transactions with cursor and offset_mode, pools — noting the sort/cursor restriction).

Validation: cargo test --all-features (107 tests) green, cargo clippy --all-targets --all-features and --no-default-features --all-targets 0 warnings, cargo fmt --check, cargo doc clean. Review loop (4-dimension adversarial reviewer fan-out) run recursively to 0 findings: cycle 1 → 11 findings, cycle 2 → 5, cycle 3 → 4, cycle 4/5 → 0.