A chained subscript WRITE on a dynamic array of dynamic arrays —
m[i][j] := v — was rejected at semantic time with "Multi-dimensional
subscript base must be a static array". Only a static-array base was
accepted in the chained-assign path; reading via an intermediate row
variable (row := m[i]; row[j] := v) worked, but the direct write did
not, making jagged 2-D+ dynamic arrays awkward to fill.
Semantic: AnalyseStaticSubscriptAssign now accepts a tyDynArray base in
the BaseExpr (chained) path and derives the element type from
TDynArrayTypeDesc.
Codegen (both backends): when the chained-write base resolves to a
dynamic array, evaluate BaseExpr (m[i]) to the inner array's data
pointer and index into it — heap indirection per level — instead of the
inline-offset static-array path. QBE adds a BaseExpr branch in the
dynarray block of EmitStaticSubscriptAssign. Native stashes the base
pointer below the spilled value on the stack and reloads it at the
base-resolve step, with matching cleanup on every Exit path (float,
record, string/class, and scalar).
Verified 2-D int, 2-D string (with overwrite), and 3-D int nesting on
both backends. Adds three dual-backend e2e tests in
cp.test.e2e.dynarray.pas and a "Dynamic-array nesting (jagged arrays)"
subsection to docs/language-rationale.adoc.
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