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fix(vertex): initialize dirFaces in vertexRotations to silence uninitialized-variable warning (#1273)

vertexRotations() declares PentagonDirectionFaces dirFaces without an
initializer, then assigns it inside a while loop that breaks as soon as
it finds the matching pentagon base cell in the pentagonDirectionFaces
table. A guard (NEVER(p == NUM_PENTAGONS)) already returns E_FAILED if
the loop runs off the end without a match, before dirFaces is read -
but some compilers can't prove that connection from the loop/break
structure alone and warn that dirFaces "may be used uninitialized".

I verified this is a false positive, not a real logic gap: the
function only reaches this branch when _isBaseCellPentagon(baseCell)
is true, and pentagonDirectionFaces is generated (by
generatePentagonDirectionFaces.c) from exactly the same NUM_PENTAGONS
pentagon base cells that _isBaseCellPentagon recognizes. So the loop
is guaranteed to find a match and assign dirFaces before break,
whenever this branch is taken. This matches the maintainer's own
assessment in the issue threads ("it should be guaranteed... We ought
to be able to add a guard here that would remove the compiler
warning").

I was not able to reproduce the warning locally with GCC 16 or Apple
Clang 17 at -O1 through -O3 with -Wall -Wextra -Wmaybe-uninitialized -
their uninitialized-variable analysis has evidently improved since
these reports (GCC ~9-11 and Clang 11 in the original issues). The fix
is still correct and low-risk for the strict toolchains (R/CRAN
packaging, embedded builds) where this keeps getting independently
re-reported across 4+ years.

Fix: zero-initialize dirFaces at declaration, with a one-line comment
(kept short per review feedback - see below). This is a pure
defensive/cosmetic change with no effect on behavior, since the
existing NEVER() guard already prevents any use of an unassigned
dirFaces at runtime.

Also adds a test (vertexRotations_allPentagonBaseCells) exercising
vertexNu... (continued)

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