[Low] Document verbosity count propagation implementation

## Description
Verbosity counts now propagate into build flags and dry-run output, with tests covering levels.

## References
- `installer/src/main.rs:85-103`
- `installer/src/builder.rs:134-136`
- `installer/tests/behaviour_cli.rs:299-332`

## Context
Raised during review of PR #93: https://github.com/leynos/whitaker/pull/93

## Priority
Low