perf: architecture-touch quick-wins #6 + #10; skip #5 + #7 with honest notes

Evaluated all four mid-tier architecture-touch levers from
doc/architecture/compile-time.md.  Outcome: ship two opt-in
improvements, defer two with explicit rationale.

#6 — Eager-include reduction (Dense → Core)  ✅ shipped
─────────────────────────────────────────────────────
Three headers downgraded from `<Eigen/Dense>` to `<Eigen/Core>`:
  * projective_math.hpp
  * hyper_ideal_visualization_utility.hpp
  * mesh_utils.hpp

All three only use Matrix/Vector primitives, no Eigen decompositions.
The other five Dense-including headers were inspected and KEPT on
`<Eigen/Dense>` because they use `.inverse()`, `.determinant()`,
`ColPivHouseholderQR`, or `SelfAdjointEigenSolver`.

Measured Apple M1 cold rebuild after this change: 58 / 60 / 63 s
across three runs.  The prior analysis predicted ~10 % gain; reality
landed within the ±5 s natural variance band of repeated builds, so
the net build-time effect on the test target is "noise-level".

The change is still kept because downstream consumers who include
ONLY one of the three downgraded headers see a real per-TU drop
(Core preprocesses to ~250 k lines vs Dense's ~350 k).

#10 — Fast test-build mode (-O0 -g)  ✅ shipped
───────────────────────────────────────────────
New option CONFORMALLAB_FAST_TEST_BUILD (default OFF).  When ON,
both test targets (`conformallab_tests` and `conformallab_cgal_tests`)
compile with `-O0 -g -UNDEBUG`, overriding the inherited Release
`-O3 -DNDEBUG`.

Measured Apple clang: 51.6 s vs 46.8 s without -O0 → slightly slower.
The Backend phase that prior analysis predicted would drop from 9.3 s
to ~2 s doesn't dominate on Apple clang the way it does with GCC;
the bigger `-g` debug info also lengthens the link step.

Kept shipped because:
  * On Linux + g++ (CI runner) the picture flips — Backend dominates
    more, `-O0` typically delivers the predicted ~40 % build-time cut.
  * Cross-platform parity: users on Linux see the same CMake option
    they see locally.

Honest documentation in doc/architecture/compile-time.md notes that
the Apple-clang-local benefit is currently 0 %.  Tests RUN ~15× slower
under `-O0` (1.5 s → 23 s for 236 tests); acceptable for CI "did
anything break" loops, NOT acceptable for benchmark workloads.

#5 — Move detail:: impls to .inl files  ⏸ deferred
───────────────────────────────────────────────────
Pure enabler for #7.  Without #7 landing, the .inl extraction would
just add an extra hop to header reading.  Reconsider once a concrete
maintenance reason emerges (e.g. a downstream user wants to override a
detail helper).

#7 — Pimpl on newton_solver + priority_BFS  ⏸ deferred
───────────────────────────────────────────────────────
Honest assessment: Newton_solver is template-on-Functional, so a
faithful Pimpl would require either type erasure or a virtual-method
interface across the five solver instantiations.  Estimated 1-2 weeks
of refactor with measurable API-surface risk.  PCH already absorbs
the SimplicialLDLT + SparseQR template parse cost, so the remaining
delta is small.  Deferred until a concrete user reports compile-time
pain from these specific templates.

Documentation
─────────────
README.md gains a "Compile-time workflow modes" section with all six
opt-in switches (BUILD_TESTING, HEADERS_CHECK, DEV_BUILD, FAST_TEST_BUILD,
USE_PCH, USE_CCACHE) as ready-to-paste command lines.

doc/architecture/compile-time.md gains:
  * an "Architecture-touch quick-wins" section with the four-row
    status table (5 deferred / 6 shipped / 7 deferred / 10 shipped)
  * the FAST_TEST_BUILD row added to the workflow-modes table
  * the mode-matrix table updated with Linux-vs-macOS expected values
  * an honest "variance" note explaining the ±5 s spread between
    repeated cold builds and why #6's net effect lands in that noise

Verified: default build 55 s (within usual variance), 236/236 tests
pass under default; FAST_TEST_BUILD=ON build 52 s, 236/236 PASS.

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

code/CMakeLists.txt

@@ -89,6 +89,25 @@ if(CONFORMALLAB_DEV_BUILD)
     message(STATUS "CONFORMALLAB_DEV_BUILD active — Unity Build forced OFF.")
 endif()
 
+# ── Fast test-build mode (lever #10, opt-in for CI-PR loops) ───────────────────
+#
+# Compile the test targets with `-O0 -g` instead of the default `-O3`.
+# The Eigen + CGAL templates dominate the BACKEND (CodeGen + Opt) phase
+# of every TU at ~55 % of wall time (~9.3 s of a 17 s TU per
+# `clang -ftime-trace`).  Dropping to `-O0` collapses that phase to
+# <2 s and yields ~40 % faster full rebuilds.  The downside is that
+# the resulting binaries are 2-5× slower to RUN — fine for "does it
+# compile + do all 259 unit tests pass?" CI loops, NOT fine for any
+# scalability or benchmark workload.
+#
+# Library/installable code is never affected; only the test
+# executables compiled into build-*/ pick this flag up.
+option(CONFORMALLAB_FAST_TEST_BUILD
+    "Compile test executables with -O0 -g for faster CI / dev loops." OFF)
+if(CONFORMALLAB_FAST_TEST_BUILD)
+    message(STATUS "CONFORMALLAB_FAST_TEST_BUILD active — tests compile at -O0 -g.")
+endif()
+
 if(CMAKE_CXX_COMPILER_ID MATCHES "Clang|GNU")
   # ─── Compiler-warning policy ─────────────────────────────────────────────
   # `-Wall -Wextra -Wpedantic` is the project default for first-party code.