quality: add code-style + CGAL-convention checkers (local-only)

Closes the gap "no code-quality / convention gate" from the structural
review.  Three new artefacts, all local-only (CI promotion deferred
until the existing tree is 100 % clean under each):

1. .clang-format — project's existing style mechanically captured
   (4-space indent, opening brace on new line for class/struct/function,
   left-aligned pointer/reference modifiers, aligned `using = ...` blocks,
   100-col loose limit, no include re-ordering — matches code/include/
   today).

2. scripts/quality/clang-format.sh — drift detector.  Dry-run mode by
   default (always exits 0); --strict to fail on drift; --fix to apply
   suggested changes in place.  Skips code/deps/ and macOS-duplicate
   files.

3. scripts/quality/cgal-conventions.py — checker for the CGAL idioms
   that clang-format/clang-tidy cannot express:
     CGAL-1  include-guard format `CGAL_<DIRS>_<FILE>_H`
     CGAL-2  every public header has a `\\file` Doxygen brief
     CGAL-3  no nested namespaces beyond the allowed set
             (CGAL::parameters, CGAL::Conformal_map, internal_np, IO)
     CGAL-4  named-parameter tag types end in `_t`; value object does not
     CGAL-5  no `using namespace ...` at file scope (header leakage)
     CGAL-6  no #define beyond CGAL_* / include-guard

   Result on the current tree: 6 CGAL public headers, 0 violations.
   The checker therefore doubles as documentation of the conventions
   we already follow.

Both are wired into scripts/quality/run-all.sh's fast subset (~5 s
combined wall time).  README.md updated to split the gates into a
"style/convention" group (cheap, run-on-every-commit material) and a
"correctness/quality" group (slow, run-before-tag material).

The reviewer-facing locked-vs-flexible.md gains another "✅ Closed"
row documenting both gates and the 0-violation baseline.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.clang-format

@@ -0,0 +1,76 @@
+# conformallab++ formatting policy
+#
+# Captures the style already present in code/include/.  Documented here
+# so clang-format can enforce it locally (scripts/quality/clang-format.sh)
+# and so new contributors get the same output their editor would on save.
+#
+# This is NOT the upstream CGAL clang-format (there isn't one published);
+# it's the style our tree already uses, mechanically extracted.
+
+BasedOnStyle: LLVM
+Language: Cpp
+Standard: c++17
+
+IndentWidth: 4
+TabWidth: 4
+UseTab: Never
+ColumnLimit: 100                  # loose; readability over hard wrap
+
+# Brace placement — matches the project tree:
+#   functions / methods → opening brace on a new line  (CGAL convention)
+#   structs / classes   → opening brace on a new line
+#   else / catch        → on the same line as the closing brace of the preceding block
+BreakBeforeBraces: Custom
+BraceWrapping:
+  AfterClass:            true
+  AfterStruct:           true
+  AfterEnum:             true
+  AfterFunction:         true
+  AfterNamespace:        false
+  AfterUnion:            true
+  AfterControlStatement: false
+  BeforeElse:            false
+  BeforeCatch:           false
+  IndentBraces:          false
+  SplitEmptyFunction:    false
+  SplitEmptyRecord:      false
+  SplitEmptyNamespace:   true
+
+# Reference & pointer modifiers attach to the type (`int& x`, not `int &x`).
+PointerAlignment: Left
+ReferenceAlignment: Left
+
+# Aligned `using = ...` blocks are intentional in the trait classes.
+AlignConsecutiveDeclarations: AcrossEmptyLines
+AlignConsecutiveAssignments: AcrossEmptyLines
+AlignTrailingComments: true
+AlignAfterOpenBracket: Align
+
+AllowShortFunctionsOnASingleLine: Inline
+AllowShortIfStatementsOnASingleLine: Never
+AllowShortLoopsOnASingleLine: false
+AllowShortBlocksOnASingleLine: Never
+AllowShortLambdasOnASingleLine: Inline
+
+# Template-related: break before each parameter when the template line
+# would otherwise exceed ColumnLimit (matches existing
+# `template <typename TriangleMesh, typename ...>` patterns).
+BreakBeforeBinaryOperators: NonAssignment
+BinPackParameters: false
+BinPackArguments: false
+AlwaysBreakTemplateDeclarations: Yes
+SpaceAfterTemplateKeyword: true
+
+NamespaceIndentation: None
+AccessModifierOffset: -4
+IndentCaseLabels: false
+
+# Includes: keep manual ordering — re-ordering can break Eigen / CGAL
+# transitive-include assumptions in subtle ways.  We just enforce no
+# accidental duplicate blank lines.
+SortIncludes: Never
+MaxEmptyLinesToKeep: 1
+KeepEmptyLinesAtTheStartOfBlocks: false
+
+# Comments: don't touch.
+ReflowComments: false