refactor(binder): route bib-lint and heading-case checks through binder

Previously two pre-commit hooks invoked scripts directly
(python3 book/tools/bib_lint.py and python3 book/tools/audit/heading_fix.py),
making them outliers among the 50+ other book hooks that already route
through ./book/binder check <group> --scope <name>. Unify the surface
by wrapping both under binder's delegated-script pattern — the same
pattern grid-tables, image-formats, external-images, and svg-wellformed
already use.

Changes:
- validate.py: add _run_heading_case and _run_bib_hygiene methods that
  delegate to the existing scripts via _delegate_script (subprocess,
  120s timeout, exit-code → ValidationRunResult translation).
- validate.py: extend the "headers" group with a new "case" scope;
  add a new "bib" group with a "hygiene" scope. Update the description
  table shown by `binder check --list`.
- .pre-commit-config.yaml: swap both hooks' entry commands from direct
  python3 invocations to ./book/binder check ... --scope ...

No behavior change at the script level — binder is a thin wrapper.
The scripts (book/tools/bib_lint.py, book/tools/audit/heading_fix.py)
remain the single source of truth for each check's rules and the
apply/fix modes (which binder does not surface, since binder check is
read-only by design). Apply modes stay invokable directly:
  python3 book/tools/audit/heading_fix.py apply
  python3 book/tools/bib_lint.py --fix <file>

Benefits:
- Single discovery surface: `binder check --list` now shows every
  validator the project runs, with no "and also these standalone
  scripts" footnote.
- Consistent debugging: `./book/binder check headers --scope case
  --verbose` works the same way as every other check.
- Easier onboarding: contributors only need to learn the binder CLI to
  understand the validation layer.

Verified:
- ./book/binder check headers --scope case → exit 0 on compliant tree
- ./book/binder check bib --scope hygiene → exit 0 on compliant tree
- Both hooks pass via pre-commit run --all-files
- Exit code propagation: deliberate bad heading → binder exit 1 with
  full diagnostic bubbled from the wrapped script.

.pre-commit-config.yaml

@@ -93,13 +93,16 @@ repos:
   # pre-existing violations — only NEW violations block the commit.
   # Regenerate baseline via: python3 book/tools/bib_lint.py --all --baseline
   # (do this only when intentionally accepting new violations).
+  # Invoked through binder for a consistent check surface — the underlying
+  # implementation still lives at book/tools/bib_lint.py (has both --check
+  # and --fix modes; only --check is wired to pre-commit).
   - repo: local
     hooks:
       - id: bib-lint
         name: "Global: Validate bibtex against §5 (semantic)"
-        entry: python3 book/tools/bib_lint.py --check
+        entry: ./book/binder check bib --scope hygiene
         language: system
-        pass_filenames: true
+        pass_filenames: false
         files: \.bib$
 
   # --- Global: Local repo guards ---
@@ -289,10 +292,11 @@ repos:
         files: ^book/quarto/contents/.*\.qmd$
 
       # Heading-case guard: enforces MIT Press H1/H2 headline-case and H3+
-      # sentence-case policy (§10.3.1 of book-prose.md). The fixer is
-      # idempotent: on the current-compliant state it proposes 0 changes,
-      # so passing files don't re-trigger. On a regression it prints the
-      # offending heading(s) with the expected sentence-case form and
+      # sentence-case policy (§10.3.1 of book-prose.md). Invoked through
+      # binder for a consistent check surface — the underlying implementation
+      # lives at book/tools/audit/heading_fix.py (check + apply modes;
+      # only check is wired to pre-commit). On a regression, binder prints
+      # the offending heading(s) with the expected sentence-case form and
       # exits non-zero. To fix a blocked commit, run:
       #   python3 book/tools/audit/heading_fix.py apply
       # Or hand-edit the reported heading(s). The fixer preserves all ten
@@ -304,9 +308,9 @@ repos:
       # named laws (Amdahl's Law), and legislation (EU AI Act).
       - id: book-check-heading-case
         name: "Book: Enforce H1/H2 headline + H3+ sentence case"
-        entry: python3 book/tools/audit/heading_fix.py check
+        entry: ./book/binder check headers --scope case
         language: system
-        pass_filenames: true
+        pass_filenames: false
         files: ^book/quarto/contents/.*\.qmd$
 
       - id: book-check-div-fences

book/cli/commands/validate.py

@@ -142,6 +142,10 @@ class ValidateCommand:
         ],
         "headers": [
             ("ids", "_run_headers"),
+            ("case", "_run_heading_case"),
+        ],
+        "bib": [
+            ("hygiene", "_run_bib_hygiene"),
         ],
         "footnotes": [
             ("placement", "_run_footnote_placement"),
@@ -416,7 +420,8 @@ class ValidateCommand:
         descriptions = {
             "refs": "References, citations, inline Python, self-ref",
             "labels": "Duplicate labels, orphans, fig-label underscores",
-            "headers": "Section header IDs ({#sec-...})",
+            "headers": "Section header IDs ({#sec-...}), H1-H5 case policy (MIT Press §10.3.1)",
+            "bib": "Bibliography hygiene — schema + canonical forms (§5)",
             "footnotes": "Placement, integrity, cross-chapter duplicates",
             "figures": "Captions, float flow, image files",
             "rendering": "Patterns, indexes, dropcaps, headings, typos, tables, ASCII",
@@ -4223,6 +4228,52 @@ class ValidateCommand:
             return ValidationRunResult(name="grid-tables", issues=[])
         return self._delegate_script(script, ["--check"] + qmd_files, "grid-tables")
 
+    def _run_heading_case(self, root: Path) -> ValidationRunResult:
+        """Enforce H1/H2 headline case + H3+ sentence case (MIT Press §10.3.1).
+
+        Delegates to `book/tools/audit/heading_fix.py check`, which encodes
+        the ten sentence-case exceptions (acronyms, hyphenated-acronym
+        compounds, digit-letter models, single-letter labels, slash acronyms,
+        CamelCase product names, lowercase API names, math spans, named
+        laws, legislation) and compound proper nouns. Running in `check`
+        mode is idempotent on a compliant tree — zero changes proposed,
+        zero output. On a regression, the script prints the offending
+        line(s) and the expected sentence-case form, exits non-zero.
+        """
+        script = (
+            Path(__file__).resolve().parent.parent.parent
+            / "tools" / "audit" / "heading_fix.py"
+        )
+        qmd_files = [str(f) for f in sorted(root.rglob("*.qmd"))]
+        if not qmd_files:
+            return ValidationRunResult(
+                name="heading-case", description="heading-case",
+                files_checked=0, issues=[], elapsed_ms=0,
+            )
+        return self._delegate_script(script, ["check"] + qmd_files, "heading-case")
+
+    def _run_bib_hygiene(self, root: Path) -> ValidationRunResult:
+        """Validate .bib files against §5 Bibliography Hygiene schema.
+
+        Delegates to `book/tools/bib_lint.py --check`, which enforces the
+        canonical schema: required fields per entry type, canonical field
+        order, quoting style, author-list rules, journal spell-out,
+        publisher canonical forms. Violations against the pre-existing
+        baseline (`book/tools/bib_lint_baseline.json`) are grandfathered;
+        only NEW violations block.
+        """
+        script = (
+            Path(__file__).resolve().parent.parent.parent
+            / "tools" / "bib_lint.py"
+        )
+        bib_files = [str(f) for f in sorted(root.rglob("*.bib"))]
+        if not bib_files:
+            return ValidationRunResult(
+                name="bib-hygiene", description="bib-hygiene",
+                files_checked=0, issues=[], elapsed_ms=0,
+            )
+        return self._delegate_script(script, ["--check"] + bib_files, "bib-hygiene")
+
     def _run_image_formats(self, root: Path) -> ValidationRunResult:
         """Validate image file formats using Pillow."""
         script = (