feat: report text linter diagnostics by module

[wkrozowski]

This PR makes lake lint --builtin-lint group saved text linter diagnostics by the module
that produced them, rather than printing one combined block under the
top-level module being linted. Each contributing submodule now gets its own
-- Text linter diagnostics in <module>: header, mirroring how the
environment-linter side already groups results.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 55cf1d71e8dc1217c559d6426571d237beb84e46 --onto 2cd98639c40d2d2a026dd599093b70f55f55ffaf. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-26 16:42:57)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 55cf1d71e8dc1217c559d6426571d237beb84e46 --onto 803553a556fd82fa1060efb0c43eda542130cb16. You can force reference manual CI using the force-manual-ci label. (2026-05-26 16:42:59)