Improving re-parse performance

Execution-Top.fs has code that repeatedly re-parses the assembler code to generate a runnable image. the repetition is needed because symbols can have arbitrary forward references. In the normal use case where only a few lines of the program have changed this should not be needed. Instructions without symbols (most of them) do not need to be re-parsed. In fact whole code segments without symbols can be memoised in a pre-parsed and relocatable form and added to the image (in `loadImage`) in a single operation.

Memoisation is a great technique for speeding up functional code since it is a local change that is provably safe (even though it uses mutable state).

Use memoisation to speed up parsing of long programs (like the sample code from the about menu).

Start by reading Execution-Top.fs, especially the repeated parsing path and loadImage. Trace how symbols and forward references affect parsing, then use the long sample program from the About menu to measure the current behavior. Done means unchanged programs avoid unnecessary re-parsing while symbol-dependent code still produces a correct runnable image.

Written by the indexing model from the issue text.