fix: reword check diagnostics in plain language

[PhilDL]

Diagnostics from balade check used phrasing like "claims no file this list still holds", "reaches the payload", "cannot stand on its own", "carries no expect". They now say what is wrong in ordinary words.

Before	After
The filegroup X claims no file this list still holds.	The filegroup X matches no file.
A fenced code block reaches the payload only at a section's top level.	Fenced code blocks are only supported at the top level of a section.
The tag x cannot stand on its own.	The tag x must be inside a parent tag.
matrix holds no table.	matrix contains no table.
Lines 3-9 fall outside f, which holds 5 lines at abc123.	Lines 3-9 are out of range: f has 5 lines at abc123.
The code reference r carries no expect.	The code reference r has no expect attribute.
Prose outside a section does not reach the payload.	Text outside a section is ignored.
The PR head moved 2 commits past the stamp, and it touches a.py	The PR head is 2 commits ahead of the stamped commit and changes a.py

32 strings across 6 files. Diagnostic codes unchanged. One test substring updated.

https://claude.ai/code/session_01EbmbcnENzTAj8KpYDoKd7U

[pkg-pr-new]

npx https://pkg.pr.new/basaltbytes/balade@150

commit: 5968be7