[Generated coverage] Support statically resolvable ConstructUsing factories in strict mode #228

Parent and baseline

Part of #227. The current main supports explicit ConstructUsing(...) factories with 1–4 mapped root values through runtime materialization. The source generator currently skips such map constructors and emits DFM011 rather than producing a strict-generated materializer. This is a concrete gap; do not change the existing runtime factory contract.

Release: future compatible version, decided through .agents/skills/semantic-versioning/SKILL.md after validating the current 3.6.1 prepared/released state. Not a retroactive 3.6.0 or 3.6.1 feature.

Goal

Generate deterministic construction for a bounded set of statically resolvable explicit factories, preserving exact runtime semantics and rejecting arbitrary closures/delegates rather than reflecting or silently falling back in strict mode.

Scope

Static factory eligibility

• Inventory each supported ConstructUsing overload and binding contract on current main (including root-property inputs, arity, construction strategy and converter/null precedence).
• Determine symbol-based eligibility for accessible static factory methods or other statically provable non-capturing forms; do not infer arbitrary expression meaning from text.
• Require deterministic callable, argument order, types, mapping paths and null behavior. Document accessibility/generic/method-group constraints.
• Retain DFM011 or a more precise, non-conflicting diagnostic for unsupported captures, dynamic delegates, helper indirection and unresolvable symbols; preserve DFM016 on conflicting construction strategies.

Generated execution

• Emit compile-time-safe direct factory invocation for eligible forms, with no MethodInfo.Invoke, DynamicInvoke, compiled-expression runtime fallback or hidden global dependencies.
• Reuse the existing generated converter/read helpers when valid; preserve the current error contract, nullable handling, profile identity and strict reader-shape rules.
• Ensure non-strict runtime factory behavior remains unchanged.

Tests, evidence, documentation

• Positive: static factory, multi-argument construction (within supported arities), nullable and converted values, immutable value-object, mapping profile where supported.
• Negative: closures/captures, inaccessible factories, ambiguous/unsupported forms, duplicate/invalid construction, missing/duplicate columns and conversion failure.
• Compare generated and runtime results/errors for equivalent supported shapes; test strict refusal to fall back.
• Document exact eligible factory forms and unsupported alternatives in generator/usage/compatibility docs; add bounded Native AOT smoke only for a shape actually certified.

Out of scope

• Parsing arbitrary lambda bodies or searching for unrelated factory names.
• Extending ConstructUsing public arity beyond 1–4.
• Runtime code generation, graph persistence, SQL generation or broad 4.0 API redesign.
• General nested construction beyond the specific factory composition needed here ([Generated coverage] Audit and close strict nested/value-object materialization gaps #229).

DoR

• Current main baseline, existing ConstructUsing runtime tests, DFM011 and DFM016 verified.
• Eligible and ineligible factory syntax/symbol shapes recorded before implementation.
• Current release preparation and SemVer policy checked; shared branch created from up-to-date main.

Acceptance

• An eligible explicit factory successfully materializes a strict-generated result without runtime reflection.
• Generated/runtime values, null rules and contextual exceptions are equivalent for supported shapes.
• Ineligible factories have deterministic generator/strict diagnostics, without false positive support claims.
• Existing ConstructUsing public/runtime compatibility remains intact.

DoD and delivery

• Generator + analyzer + strict/runtime regression tests added, with positive/negative cases.
• XML/PublicAPI baselines updated only for justified public additions; no breaking change introduced implicitly.
• Performance/Native AOT evidence recorded for newly claimed behavior.
• Relevant EN/PT-BR documentation and compatibility limits updated.
• Commit/push to shared roadmap/generated-coverage-followups branch; do not open an intermediate PR. [Generated coverage] Certify strict support matrix, compatibility and cumulative delivery #232 opens the cumulative PR.

Dependencies: parent #227; #229 follows where nested factories need this capability. #230 is closed and not a blocker.