v0.1.8.10 Substrate, Accounting, And B2 Sweep Opt-In

v0.1.8.10 closes the v0.1.8.x single-core substrate round.

Highlights:

• Adds matrix-canonical strategy accessors (ctx$idx(), ctx$vec, and bulk feature reads).
• Moves FIFO lot accounting into fold-owned state while preserving canonical event evidence and durable replay/readback.
• Adds scoped public memory-backed sweep opt-in: ledgr_sweep(..., compiled_accounting_model = 'spot_fifo').

Compatibility:

• Default execution remains canonical R (compiled_accounting_model = NULL).
• Durable ledgr_run(..., compiled_accounting_model = 'spot_fifo') still fails closed until a separate durable compiled-integration gate lands.
• The B2 path is spot-asset FIFO only; it is not a general compiled fold core or non-spot accounting engine.