forms: recurse one level into nested-aggregate members' schemas

[Yaraslaut]

Summary

• Issue Form generation cannot render nested aggregates (flat actions only) #25 explicitly asked whether flat-only form generation was a deliberate boundary or a gap worth closing, and noted it was sketching open questions, not a design. Confirmed with the repo owner that recursive generation should be implemented before starting.
• mergeSchemaExtras previously annotated (x-order, required, title, Quantity/Choice/widget hints) only an action's own top-level members. A member that was itself a reflectable aggregate — a nested struct, or std::vector<Sub> — got none of that on its own sub-members, matching the issue's exact repro (Specimen/replicates landing in $defs with no annotations and no required).
• Recurses one level into a nested-aggregate member's own schema, reusing the same per-member annotation logic (factored into a shared annotateBasicMemberProperty helper so both the top-level pass and the nested pass apply identical rules).
• Handles both schema shapes glaze produces for a nested aggregate (discovered empirically while implementing — not something either the issue or the existing docs called out): a shared $defs entry referenced by $ref when the nested type is used 2+ times anywhere in the schema, or the object schema inlined directly into the property when it's used exactly once. annotateNestedAggregateRef resolves whichever form applies before recursing.
• Capped at exactly one level (a nested aggregate's own nested-aggregate members stay unannotated) and computed fields/formLayout/fieldSpans/formRules remain top-level-only — matching the depth-limit and scope suggestions in the issue itself.
• Purely additive: an action with no nested-aggregate member has nothing to trigger on, so its schema is byte-for-byte unchanged — confirmed by the full existing suite (every prior schema-generation test) passing unmodified.
• Explicitly out of scope (per the issue's own framing, which separates this from the annotation-reach gap): render-side add/remove UI affordances for a repeated-aggregate array. That's a downstream renderer concern.
• docs/spec/forms/forms.md updated: replaces the old "flat actions only" scope note with a new "Nested aggregates (one level)" section describing both schema shapes, the depth cap, and what stays top-level-only.

Test plan

• New tests/test_nested_forms.cpp: flat top-level fields unaffected; single nested member and std::vector<Sub> member annotated correctly in both the deduplicated ($ref/$defs) and inlined forms; depth cap (level-2 nesting stays unannotated); idempotence when two members share the same nested type.
• Full suite: ./build/tests/morph_tests — all 818 test cases / 8347 assertions pass, including every pre-existing schema-generation test (test_quantity_forms.cpp, test_forms_conformance_corpus.cpp, etc.) unmodified.

Closes #25

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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