Phase 8b-Lite: pipe-operator chaining for named parameters

Adds `operator|` in `namespace CGAL` so package-local named parameters
can be combined left-to-right without modifying CGAL upstream:

    auto p = CGAL::parameters::gradient_tolerance(1e-12)
           | CGAL::parameters::max_iterations(500)
           | CGAL::parameters::output_uv_map(uv);
    CGAL::discrete_conformal_map_euclidean(mesh, p);

Why not the canonical `.a().b().c()` syntax
───────────────────────────────────────────
CGAL's standard chaining mechanism requires registering each named
parameter as a member function on `Named_function_parameters` via the
`CGAL_add_named_parameter` macro in
`CGAL/STL_Extension/internal/parameters_interface.h` — a vendored
upstream file that conformallab++ deliberately treats as read-only.

Adding member-function chainers for our package-local tags would
require either forking CGAL or modifying the vendored copy.  Neither
is acceptable for a library that wants to remain portable across
future CGAL releases.

The pipe-operator achieves the same compositional semantics via a
free function in `namespace CGAL` (so ADL finds it for
`Named_function_parameters` operands).  Implementation: rebuild the
right-hand-side `Named_function_parameters` with the left-hand-side
as its `Base`, producing an indistinguishable chain that every entry
function accepts unchanged.

Implementation: `code/include/CGAL/Conformal_map/internal/parameters.h`
lines 158-187.  The operator is constrained to right-hand-sides with
`No_property` base (i.e. fresh single-parameter packs from the helper
functions), so it never collides with any future CGAL operator on the
same type.

Tests (2 new, total Phase-8b-Lite suite 15 → 17)
────────────────────────────────────────────────
* CGALPhase8bLite.NamedParamPipe_MultipleParamsTakeEffect
    Chain three parameters; verify all three take effect (tight
    tolerance respected + UV pmap populated + iteration cap honoured).
* CGALPhase8bLite.NamedParamPipe_TwoParams
    Chain two parameters; verify max_iterations(0) blocks the loop
    even when combined with another param.

Full CGAL suite: 234/234 PASSED, 0 SKIPPED (was 232).
Total: 257/257 PASSED, 0 SKIPPED (was 255).
scripts/check-test-counts.sh: OK.

Documentation updates
─────────────────────
* doc/tutorials/add-output-uv-map.md §3.4: "Current limitation: no
  chaining" → "Chaining: use the pipe operator `|`".  Explains why
  CGAL's `.member()` syntax isn't available and shows the `|`
  workaround with a working code example.
* doc/architecture/locked-vs-flexible.md §8: chaining now flagged as
  shipped via pipe; recommended posture says `.member()` chaining
  only if a user pushes for the CGAL-canonical syntax.
* doc/roadmap/porting-status.md §5: API limitations table updated.
* doc/api/tests.md: CGALPhase8bLite row 15 → 17, total 232 → 234.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

doc/architecture/locked-vs-flexible.md

@@ -150,12 +150,13 @@ default trait per algorithm family.
 |--|--|
 | Status         | 🟢 opportunistic |
 | Locked since   | Phase 8 MVP (2026-05-19) |
-| Current state  | Six tags: `vertex_curvature_map`, `fixed_vertex_map`, `gradient_tolerance`, `max_iterations`, `output_uv_map`, `normalise_layout`.  No chaining yet. |
-| Cost to extend with chaining | ~2 days.  Generate CGAL-style member-function chainers via macros (CGAL has a `CGAL_add_named_parameter` macro for this). |
+| Current state  | Six tags: `vertex_curvature_map`, `fixed_vertex_map`, `gradient_tolerance`, `max_iterations`, `output_uv_map`, `normalise_layout`.  Pipe-operator `|` chaining shipped (in lieu of `.member()` chaining which would require CGAL upstream modifications). |
+| Cost to extend with `.member()` chaining | ~2 days IF CGAL upstream is forked / patched; otherwise the pipe-operator workaround is the maintainable path. |
 | When to revisit | At any time; this is the lowest-risk change in the codebase.  Tutorials [`add-output-uv-map.md`](../tutorials/add-output-uv-map.md) §4 explains the mechanism. |
 
-**Recommended posture:** add chaining when the first user complains.  Or
-when adding the 8th-10th named parameter (still 6 today — manageable).
+**Recommended posture:** the pipe-operator `|` is already shipped and
+sufficient.  Add `.member()` chaining only if a concrete user pushes for
+the CGAL-canonical syntax AND we are willing to fork CGAL upstream.
 
 ---
 
@@ -229,7 +230,7 @@ each new public function.
 | 5. Newton + line search + SparseQR      | 🟡 semi-fixed   | ~2 days              |
 | 6. Eigen back-end                       | 🔴 load-bearing | ~2 weeks             |
 | 7. Strategy C (per-functional traits)   | 🟡 semi-fixed   | ~1 week              |
-| 8. Named-parameter mechanism            | 🟢 opportunistic | ~2 days (chaining)   |
+| 8. Named-parameter mechanism            | 🟢 opportunistic | pipe ✅; .member() ~2 days |
 | 9. Property-map name conventions        | 🟢 opportunistic | ~1 hour              |
 | 10. GTest, not CGAL test format         | 🟡 semi-fixed   | ~1 week              |
 | 11. MIT license                         | 🔴 load-bearing | organisational       |