Add client methods for 4 registry endpoints with no SDK exposure (index_price, margin_borrow, liquidation_stats, listings_historical)

[martinkersner]

Follow-up to #123 (PR #125). The registry-param coverage audit (tests/test_endpoint_param_coverage.py) surfaced 4 endpoints declared in datamaxi/_endpoints.py that have NO client method at all — every one of their params is unreachable from the SDK. These were allow-listed in the audit test (with NOTE FOR HUMAN REVIEW) rather than exposed, because adding them is not a param forward-through: each needs a brand-new client method (naming, return-shape handling, docs, tests) — a product/design decision out of scope for the coverage audit.

Two of these were not even in #123's original heuristic list (index_price, margin_borrow).

Endpoints missing a client method (with their registry params)

• index_price — asset, from, to, interval
• margin_borrow — asset
• liquidation_stats — window, exchange, min_volume_usd
• listings_historical — refresh

Decisions needed per endpoint

• Do we want to expose it at all? (Some may be internal / front-api-adjacent / not intended for the public SDK.)
• Which client class does it belong on, and what method name?
• Return shape: raw page vs paginated pager vs DataFrame option, consistent with sibling methods.
• Docs + wire-level forwarding tests.

Done when

• Each of the 4 endpoints is either (a) given a client method that forwards its registry params (with tests + docs), or (b) explicitly documented as intentionally SDK-excluded.
• The corresponding allow-list entries in tests/test_endpoint_param_coverage.py are removed (for exposed ones) or their rationale updated (for intentionally-excluded ones), so the audit stays honest.

Ref: PR #125, tests/test_endpoint_param_coverage.py _ALLOWLIST.