mesh: one transfinite interpolation for mapped hex and mesh.block; stable oracle report (#52)

* mesh: one transfinite interpolation for mapped hex and mesh.block; stable oracle report

mesh.block's lattice is now built from the structured hex mesher's
transfinite code: faces by quad::transfinite_grid, the interior by the new
quad::transfinite_volume (Gordon-Thiel, blended by the block's graded
parameters), which hex::mapped now calls too. 2D patches are the
transfinite_grid of their sides. Edge curves return their corners exactly.

Oracle: a zero-reference quantity (rounding noise) keeps its absolute
tolerance as its fixture band, and docs/oracle.md shows such noise and
relative errors inside the fixture band as bounds, so the report no longer
changes with the order of floating-point operations (it failed the
'docs/oracle.md is current' check on main after #50). Scordelis-Lo moved by
1.4e-9 relative (an ill-conditioned thin shell of solids: sub-ulp node
changes) and is re-blessed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* simplify: shared face filling and lattice index; faces blended like the volume; exact ends for every edge

quad::transfinite_faces fills a block's six faces (with a per-face hook
that hex::mapped uses to project onto its B-rep faces) and
quad::lattice_index fixes the lattice layout for all three users.
transfinite_grid_blended blends faces by the block's graded parameters, so
faces and interior use one parametrisation and mesh.block reproduces its
earlier Gordon-Hall lattice (the Scordelis-Lo fixture is back to its
blessed value). Straight edges also return their corners exactly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

tests/oracle/patch-hex20/fixture.json

@@ -6,7 +6,7 @@
       "value": 1.0000000000000033
     },
     "sxy": {
-      "band": 1.0000000000005828e-12,
+      "band": 1e-9,
       "value": 5.826462044860843e-16
     },
     "syy": {