docs: record what phase 8 built, and what is left for a person across all nine

The plan's last phase becomes a record, and the document gains a closing status:
every count the audit was written to move, from the state in "Where this app
stands" to what `m3-audit.sh` reports today, and a gathered list of what a person
still has to look at — the eight screens with competing filled buttons, the two
list-detail families the pane work did not reach, the container transform, desktop
keyboard traversal, and the avatar picker's selected state.

**The audit caught the previous commit.** `ThemeGallery` added eight string
literals in composables, taking the count 39 -> 47, which is exactly the drift the
budget exists to notice. They are sample text — the words are chosen to be words,
so that colour pairings can be looked at — and putting them in the catalogue would
add eight entries no screen shows and a translator would have to be told to
ignore.

So the audit grows a third exemption marker beside `m3-color-exempt` and
`m3-spacing-exempt`: `m3-string-exempt`, per file rather than per line, because
the exemption is a property of what the file is for and eight markers down one
gallery would say less than one at the top of it. Back to 39, and the report now
says how many files are exempt so the mechanism cannot be used quietly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

composeApp/src/commonMain/kotlin/press/mantra/compose/ui/theme/ThemeGallery.kt

@@ -1,3 +1,7 @@
+// m3-string-exempt: the words in this file are sample text for looking at colour
+// pairings, not UI text. Nobody navigates here and nothing here is translated;
+// putting these eight strings in the catalogue would add eight entries that no
+// screen shows and that a translator would have to be told to ignore.
 package press.mantra.compose.ui.theme
 
 import androidx.compose.foundation.layout.Arrangement