TVdP tree node type (#39)

Replace 4 parallel integer arrays (info, up, left, right) with a single
fitpack_tree_node derived type for the triply-linked binary tree used by
the Theil-Van de Panne convexity-constrained spline fitting procedure.

- Add fitpack_tree_node type with 4 integer fields
- Refactor fpcosp: tree is now a local automatic array (4 fewer arguments)
- Refactor fpadno, fpdeno, fpfrno, fpseno to take tree(maxtr)
- Update callers cocosp and fpcoco: remove tree workspace from iwrk
- Reduce iwrk requirement by 4*maxtr integers

Author: perazz

docs/refactoring/00_refactoring_summary.md

@@ -77,22 +77,20 @@ Removed unused variables: `cos`, `sin`, `piv`, `i3`, `iq`, `ic` from `fpgrre`;
 `cos`, `sin`, `piv`, `i2`, `i3` from `fptrnp`;
 `co`, `si`, `piv`, `i2`, `ij` from `fptrpe`.
 
----
-
-## Remaining Plan
-
-Each item below is a separate PR. PRs are ordered by dependency; documentation (PR 10+) can
-proceed in parallel with any code PR.
+### PR 9: TVdP Tree Node Type (DONE)
 
-### PR 9: Back-Substitution Interface
-
-**Scope**: Unify `fpback` and `fpbacp` under a generic `fp_backsolve` interface
-**Occurrences**: ~25 call sites
-**Difficulty**: Low — clean abstraction, no logic changes
-**Book references**: Ch. 4 Eq. 4.14 (R₁·c = z₁), Ch. 5 Eq. 5.15 (R₁*·c = z₁*)
+**Changes**: Introduced `fitpack_tree_node` derived type with 4 integer fields (`info`,
+`up`, `left`, `right`) replacing 4 parallel integer arrays for the triply-linked binary
+tree used by the Theil-Van de Panne (TVdP) convexity-constrained spline fitting procedure.
+Refactored `fpcosp` to allocate the tree locally (removing 4 array arguments from its
+signature), and updated all 4 helper routines (`fpadno`, `fpdeno`, `fpfrno`, `fpseno`) to
+take a single `tree(maxtr)` array. Updated callers `cocosp` and `fpcoco` to remove tree
+workspace slicing from `iwrk`, reducing `iwrk` requirement by `4*maxtr` integers.
 
 ---
 
+## Remaining Plan
+
 ### PR 10+: Doxygen + MathJax Documentation
 
 **Scope**: Add structured documentation headers to all core routines in `fitpack_core.F90`