docs: implementation plan for the rule builder

Twelve tasks against the design approved today, TDD throughout: RuleQuery
comes first as a value type with no widget dependency, tested on exact
strings, and the dialog is wired to it only once parsing and compiling
round-trip.

Two tasks carry the guarantees the design was shaped around rather than
merely testing behaviour. Task 7 round-trips every query shape present in
the live rules file and pins it with a mutation check, since a compile
that differs by one paren would rewrite a file a second tool reads. Task
6 pins whole-query rejection, because a parser that salvages the part it
understands is how a not clause goes missing and a live mail filter
silently widens.

Queries in the tests are generic placeholders. The shapes are what is
under test and they survive substitution intact.

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