Preserve and print Ledger-style lot annotations

[simonmichael]

• dev: add a CostBasis field to Amount
• feat: preserve Ledger-style lot info, and show it in print's json & txt output
• feat:print: show lot info in beancount output; update docs

[simonmichael]

It seems to work; any review is welcome.
I haven't tested for time/space impact of the new field in Amount.

[simonmichael]

I tested performance with

j bench -w hledger,hledger-cb -n2 -N2
j bench-throughput hledger; j bench-throughput hledger-cb
j perftest

and all looks normal. Also, I've added

• examples/investing/export-lots-workflow, a how-to for lot reporting with other apps
• in the same directory, claude-generated general how-tos for gains reporting with ledger/beancount/rustledger