De-reflect + DRY the array/enumerable converters (P4)

FIX-PLAN.md

@@ -2,12 +2,12 @@
 
 _Derived from the 2026-07-10 three-part review (architecture · tests · performance). Every finding below was verified against source with file:line. Work items are self-contained and ordered so they can be implemented one at a time._
 
-## Progress (2026-07-12)
+## Progress (2026-07-13)
 - **Done & merged:** B1, B2, B4, B5, B9 + T1, T2 (PR #8) · BindingContext test (#10) · D3 namespace typo (#11) · T3 DI integration tests (#12) · solution rename (#13) · **A2 naming → ExistForAll (#15)** · **P0 benchmark harness (#16)** · **P1 provider cache + C3 decided/implemented (#17)** · **P2 memoize `ExtractTypeProperties` + `HashSet` dedup (#18)** · **docs tutorials refresh (#20)** · **Q1–Q4 perf quick wins + M1 collision fix + micro-benchmarks (#21)**.
 - **Q1–Q4 proven** via isolated micro-benchmarks (macro `ScanBenchmark` can't resolve them): Q1 2.7× / 64 KB→88 B · Q3 2.65× / 152 B→0 · Q4 32× / 224 B→0. **Q5 was already resolved by B4.** **M1** (code-review finding): namespace-qualify the generated impl name in the generator only — `GetNormalizeInterfaceName` also backs the section name. Suite → **56 per TFM**.
-- **Merged since:** **benchmark-tracking CI (#22)** — BDN on push/PR, gates PRs on **allocation** regressions (>10%) via github-action-benchmark on `gh-pages`; time informational. · **session-wrap docs (#23)**.
-- **In flight:** **P3 — cached "settings plan"** (`SettingsPlan` per type: section name resolved once + lazily, per-property key/default/converter precomputed, `[SettingsProperty]` read **once** per property). Warm re-populate **−55–61%** allocations (50 props 15,681→6,816 B); gated `ScanBenchmark` **≈flat (−0.4%)** — after a code+perf review pass, consolidating the 3× attribute read more than paid for the plan objects. The emitted/compiled setter was built and measured but **reverted** — it regressed the gated cold scan **+25%** for **no** warm gain (net10 reflective `SetValue` is already allocation-free for the args). A new gated `PlanPopulateBenchmark` tracks the warm path.
-- **Next:** P4 (de-reflect array/enumerable converters) → P5 (resolve config section once per type).
+- **Merged since:** **benchmark-tracking CI (#22)** — BDN on push/PR, gates PRs on **allocation** regressions (>10%) via github-action-benchmark on `gh-pages`; time informational. · **session-wrap docs (#23)** · **P3 — cached "settings plan" (#24)** — `SettingsPlan` per type (section name once+lazy, key/default/converter precomputed, `[SettingsProperty]` read once). Warm re-populate **−55–61%** (50 props 15,681→6,816 B); gated `ScanBenchmark` **≈flat (−0.4%)**. Reviewed by code+perf agents; emitted/compiled setter reverted (regressed the gated cold scan for **no** warm gain — net10 `SetValue` is already alloc-free). New gated `PlanPopulateBenchmark` tracks the warm path.
+- **In flight:** **P4 — de-reflect + DRY the array/enumerable converters** = **PR #25 open, code-reviewed clean** (branch `perf/p4-dereflect-converters`; awaiting CI + merge). New shared `CollectionTypeConverter` builds results via `Array.CreateInstance` + indexed fill and selects the element converter by walking the concrete `LinkedList` (struct enumerator) — no `List<T>`+reflected `Enumerable.ToArray`, no `First` closure. `TypeConverter.CreateNullResult` de-reflected too (`Array.CreateInstance(t,0)` for `Enumerable.Empty<T>()`). Proven via new gated `ConvertArrayBenchmark`: **1.33 KB→688 B (−49%), 1,247→219 ns (5.7×)**. Suite **68 per TFM** (+12 collection-converter parity tests). Branch also carries the pre-P4 doc refresh + post-P3 style tweaks to `TypeConverter.cs`/`TypeExtensions.cs`.
+- **Next:** P5 (resolve config section once per type) → optional P3b (tiered/lazy compiled setter, only if set *time* shows in a profile).
 - **C3 — DECIDED (option 2):** cache in the provider only; Core `SettingsBuilder.GetSettings` unchanged; no reload. See #17.
 - **Held — do NOT delete (feature work coming):** D1 Validations (reconcile with the `validate-settings` branch) · D2 EqualityCompererCreator.
 - Running status lives in `SESSION-HANDOFF.md`.
@@ -58,7 +58,7 @@ _Derived from the 2026-07-10 three-part review (architecture · tests · perform
 - [x] P2 · Memoize `ExtractTypeProperties` + fix O(n²) dedup · Sev High · Eff S
 - [x] P3 · Cached “settings plan” — hoist section (lazy) + keys, precompute/cache converters, plan per type. Reflective `SetValue` kept; compiled setter deferred (regressed the gated cold scan for no warm gain) · Sev High · Eff L
 - [x] Q1–Q5 · Quick wins (GetEnumerator, OrdinalIgnoreCase, env-binder, type-cache; Q5 dead ctor checks already done by B4)
-- [ ] P4 · De-reflect array/enumerable converters · Sev Med · Eff M
+- [x] P4 · De-reflect + DRY array/enumerable converters (shared `CollectionTypeConverter`; `Array.CreateInstance` + manual converter walk; `CreateNullResult` de-reflected) — **1.33 KB→688 B, 5.7×**; branch `perf/p4-dereflect-converters` · Sev Med · Eff M
 - [ ] P5 · Resolve config section once per type, not per property · Sev Med · Eff M
 
 **Phase 6 — Architecture strategy**
@@ -220,8 +220,9 @@ The populate loop (`src/Core/ExistForAll.SimpleSettings/ValuesPopulator.cs:36-55
 - **Q4** `SettingsClassGenerator.cs:37` string-keyed assembly `GetType(...Replace("+"…))` per generate → `Dictionary<Type, Type>` cache.
 - **Q5** `BindingContext.cs:31-32` dead null-checks per allocation (also covered by B4).
 
-### P4 · De-reflect array/enumerable converters  — Sev Med · Eff M
-`TypeConverter.cs:22-23` (empty enumerable via `Enumerable.Empty` `MakeGenericMethod().Invoke()`), `ArrayTypeConverter.cs:40,48-52` (`Activator.CreateInstance(List<>)` + reflected `ToArray`), `EnumerableTypeConverter.cs:40`. Use `Array.Empty<T>()` factories, `Array.CreateInstance(elementType, n)` + indexed assignment, and cache any unavoidable `MethodInfo`/generic instantiations per element type. (`ArrayTypeConverter`/`EnumerableTypeConverter` are near-duplicates — DRY them here.)
+### P4 · De-reflect + DRY array/enumerable converters  — Sev Med · Eff M · **DONE (branch `perf/p4-dereflect-converters`)**
+Was: `TypeConverter.cs` (empty enumerable via `Enumerable.Empty` `MakeGenericMethod().Invoke()`), `ArrayTypeConverter` (`Activator.CreateInstance(List<>)` + reflected `Enumerable.ToArray` `Invoke`), `EnumerableTypeConverter` (`Activator.CreateInstance(List<>)`), both selecting the element converter with LINQ `First` (boxes the `LinkedList` enumerator + a closure).
+Now: a shared `CollectionTypeConverter` base implements `Convert` once — normalize the value to an array (split delimited string / passthrough / wrap scalar), select the element converter by walking the concrete `LinkedList` (struct enumerator, no boxing/closure), then fill an `Array.CreateInstance(elementType, n)` by index. `ArrayTypeConverter`/`EnumerableTypeConverter` are now thin subclasses differing only in `CanConvert` + element-type extraction; both return `T[]` (safe — `IsEnumerable()` matches only `IEnumerable<T>`, which a `T[]` satisfies). `CreateNullResult` uses `Array.CreateInstance(t,0)` instead of the `Enumerable.Empty<T>()` reflection. **Proof (`ConvertArrayBenchmark`, gated): 1.33 KB→688 B (−49%), 1,247→219 ns (5.7×).** 12 parity tests in `Conversion/CollectionConversionTests.cs` (int/string/enum/DateTime/Uri elements, empty-entry removal, custom delimiter, default passthrough, unbound→empty `T[]`, `T[]`-not-`List` guard, bad-element negative). Residual 688 B = irreducible split-substrings + element boxing + result array (shared by old & new). **Code-reviewed clean** (dotnet code-reviewer verified all parity claims; the enum/DateTime/Uri + null-path + negative tests were its suggestions — partially closes T6).
 
 ### P5 · Resolve config section once per type  — Sev Med · Eff M
 `ConfigurationBinder.BindPropertySettings` (`ConfigurationBinder.cs:25-33`) calls `_configuration.GetSection(...)` **per property**; the section is constant per type. Resolve the `IConfigurationSection` once per (type, section) — pass section context via the plan (P3) or cache per section string. Touches the binder/context contract.