Configurable per-depth separators for nested array cells

[Copilot]

The plain-text array parser preferred | and otherwise fell back to #, making id#count|id#count and a single 801000#2 parse with different rules — the latter being silently split into [[801000],[2]] instead of [[801000, 2]]. There was no way to model nested arrays unambiguously.

Changes

• ParseOptions.ArrayNestedSeparators: new string option where the N-th character is the delimiter for the N-th array nesting level (e.g. "|#-" → depth-1 |, depth-2 #, depth-3 -). Empty preserves legacy |/# priority behavior.
• ArrayDataProcessor: WriteDelimitedToStream now picks the separator by current depth and recurses with a [ThreadStatic] depth counter incremented around inner-array element writes. When the configured string is shorter than the current depth, the last character is reused.
• CLI (DataTables.Generator): adds --arrayNestedSeparators to both ExportAll and ExportOne, plumbed through ParseOptions.
• DataTableProcessor: applies ParseOptions.ArrayNestedSeparators to the static config at construction; empty/null normalizes to legacy mode.
• Tests: ArrayNestedSeparatorsTests covers the singleton case, multi-entry, triple-nested, separator overflow reuse, and the empty-config fallback.
• README: documents the new flag.

Example

# id#count|id#count  →  depth-1 '|', depth-2 '#'
dotnet dtgen -i ./Tables -co ./Generated -do ./Data \
  --arrayNestedSeparators "|#"

With array<array<int>>:

Cell value	Before	After
1#2|3#4	[[1,2],[3,4]] (by accident)	[[1,2],[3,4]]
801000#2	[[801000],[2]] ❌	[[801000,2]] ✅