[stack 20/20] spec(gazprea): permit nested aggregates and n-d arrays

[Sir-NoChill]

Open the type system to the settled "arbitrary nesting and arbitrary
structs" decision. Vectors, structs, tuples, and arrays may now hold any
storable element or field type, and fixed-size arrays generalize from the
two-dimensional matrix ceiling to arbitrary rank (T[n1]...[nk]). This
reverses the vector element-type restriction and the bans on nesting
structs/tuples inside structs and tuples.

A single authoritative rule lives at ssec:storable_types (types.rst):
everything except streams is storable; nesting is unbounded but must be
acyclic through value types; recursion is legal only through a vector,
the sole point of indirection. Each per-type page now defers to it, and
array.rst's element-type list is widened to match.

The >=2 field/element arity requirement is unchanged. Rank-agnostic
operations (a shape interface, n-d matrix multiply, broadcasting) are
left to a follow-up revision.

Refs: #132 #106 #82 #71 #101 #86
Assisted-by: Agent (claude) ai@blobfish.icu

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[Sir-NoChill]

Collapsed into #139 — a single consolidated PR of the whole stack (all 40 commits preserved). The flagged questions from this PR are aggregated in #139's description; this branch and thread remain here for reference. Closing in favour of #139.