adapter.py

Execution adapters (§1.1.2) and the FROZEN adapter registry (§1.1.3).

    "Execution sits behind an adapter interface. `BacktestAdapter` is the first
     implementation. A broker adapter is a later implementation of the same
     interface, not a rewrite."

    ⛔ "Until then the adapter registry ships with exactly one entry and no way
       to add another at runtime."

That prohibition is enforced here, not merely observed: `_ADAPTERS` is a
`MappingProxyType`, there is no `register()`, no entry-point scan, no plugin
path, and `make_adapter` rejects any name that is not the one entry. A live
adapter requires the hardening surface (kill switch, governor, session gates,
prop rules) to exist on this side first; when it does, it is added HERE, in
source, in a reviewed diff.

THE FILL CONVENTION -- the reason the engine exists
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    buy  fills at the ASK      sell fills at the BID

The suite has measured that its replay fills are unfaithful (0.00% of trades
print inside the spread) and that the P&L *is* the crossing cost. Nothing in
this file ever computes a mid price for a fill, and
`tests/test_fills.py::test_a_mid_price_fill_would_fail_this_test` fails loudly
if that ever changes.

    market / stop fills  -> taker, crossing price, `slippage_ticks` ADVERSE
    limit fills          -> maker, AT the limit price, no price slippage
                            (a limit slips in WHETHER it fills, not at what price)
    stop fills           -> `min(stop, bid)` long / `max(stop, ask)` short, so a
                            gap through the stop fills at the gapped price, never
                            better than the stop