pin ruff and state the lint rules explicitly (#10)

CI has been red on unchanged code. `ruff>=0.6` is unpinned, and ruff's *default*
rule set widened in 0.16, so CI (which installed 0.16.4) reported 22 findings
across 12 files while a local `.venv` on 0.15.12 reported none.

Declare `[tool.ruff.lint] select` as ruff's pre-0.16 defaults -- the rules this
project has actually been checked against -- so the selection is a project
decision rather than whatever the installed ruff happens to default to, and pin
ruff to `>=0.16,<0.17` so local and CI agree on the version.

No source changes: main is clean under 0.16.4 once the selection is stated.
Adopting the wider rule set is a separate question, left for its own change.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: jbloom

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Pin `ruff` to `>=0.16,<0.17` and state the lint rules explicitly with
+  `[tool.ruff.lint] select`. Ruff's *default* rule set widened in 0.16, which
+  turned CI red on unchanged code; the selection is now a project decision
+  rather than whatever the installed ruff defaults to. Development-only, no
+  effect on the package.
+
 ## [0.3.0] - 2026-05-19
 
 ### Added

pyproject.toml

@@ -36,7 +36,7 @@ Issues = "https://github.com/jbloomlab/tree-annotated-plot/issues"
 [project.optional-dependencies]
 dev = [
     "pytest>=8",
-    "ruff>=0.6",
+    "ruff>=0.16,<0.17",
     "black>=24",
 ]
 docs = [
@@ -58,3 +58,10 @@ target-version = ["py311"]
 
 [tool.ruff]
 target-version = "py311"
+
+[tool.ruff.lint]
+# Ruff's default rule set is not stable across releases -- 0.16 broadened it,
+# which turned CI red on unchanged code. State the rules we lint against so a
+# ruff upgrade changes them only when we say so. These are ruff's pre-0.16
+# defaults, which is what this project has been checked against all along.
+select = ["E4", "E7", "E9", "F"]