plan(HdrHistogram#119): initial brief from issue

Author: leecampbell-codeagent

plan/planning/brief.md

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+# Brief: Issue #119 — Add support for all currently supported .NET frameworks (net9.0, net10.0)
+
+## Summary
+
+The library currently targets `net8.0;netstandard2.0`.
+As of March 2026, .NET 9 (STS) and .NET 10 (LTS) are both in active support alongside .NET 8 (LTS).
+This issue adds `net9.0` and `net10.0` as target frameworks across the library, test, examples, benchmarking, and CI projects.
+No code logic changes are expected — the existing `#if NET5_0_OR_GREATER` guard in `Bitwise.cs` already covers all three modern targets.
+The spec file `spec/tech-standards/build-system.md` must also be updated to reflect the new targets.
+
+## Affected Files (confirmed by exploration)
+
+| File | Current value | Required value |
+|------|--------------|----------------|
+| `HdrHistogram/HdrHistogram.csproj` | `net8.0;netstandard2.0` | `net10.0;net9.0;net8.0;netstandard2.0` |
+| `HdrHistogram.UnitTests/HdrHistogram.UnitTests.csproj` | `net8.0` | `net10.0;net9.0;net8.0` |
+| `HdrHistogram.Examples/HdrHistogram.Examples.csproj` | `net8.0` | `net10.0` |
+| `HdrHistogram.Benchmarking/HdrHistogram.Benchmarking.csproj` | `net8.0` | `net10.0;net9.0;net8.0` |
+| `.github/workflows/ci.yml` | `dotnet-version: 8.0.x` | `8.0.x`, `9.0.x`, `10.0.x` (multi-line) |
+| `spec/tech-standards/build-system.md` | Documents `net8.0;netstandard2.0` | Update to reflect new targets |
+
+## Conditional Compilation
+
+Only one `#if` guard exists in the main library:
+
+- `HdrHistogram/Utilities/Bitwise.cs`: `#if NET5_0_OR_GREATER` — uses `System.Numerics.BitOperations.LeadingZeroCount()`.
+
+This guard already applies correctly to net8.0, net9.0, and net10.0.
+No new `#if` directives are required.
+No `#if NET9_0_OR_GREATER` or `#if NET10_0_OR_GREATER` optimisations have been identified as necessary for this changeset.
+
+## Acceptance Criteria
+
+- [ ] `HdrHistogram.csproj` targets `net10.0;net9.0;net8.0;netstandard2.0`
+- [ ] `HdrHistogram.UnitTests.csproj` targets `net10.0;net9.0;net8.0`
+- [ ] `HdrHistogram.Examples.csproj` targets `net10.0`
+- [ ] `HdrHistogram.Benchmarking.csproj` targets `net10.0;net9.0;net8.0`
+- [ ] CI installs .NET SDK 8.0.x, 9.0.x, and 10.0.x
+- [ ] `dotnet build -c Release` succeeds for all target frameworks
+- [ ] `dotnet test` passes on all three modern runtimes (net8.0, net9.0, net10.0)
+- [ ] `dotnet pack` produces a NuGet package containing assemblies for all four targets
+- [ ] No regressions — all existing tests pass on all targets
+- [ ] `spec/tech-standards/build-system.md` updated to reflect the new target frameworks
+
+## Test Strategy
+
+No new tests need to be written.
+The existing test suite in `HdrHistogram.UnitTests/` provides full coverage.
+Multi-targeting the test project is sufficient: `dotnet test` automatically runs all tests against each `<TargetFrameworks>` entry.
+The CI pipeline, once updated to install all three SDKs, will exercise net8.0, net9.0, and net10.0 in a single `dotnet test` invocation.
+
+Verification locally:
+
+```bash
+dotnet build HdrHistogram/HdrHistogram.csproj -c Release
+dotnet test HdrHistogram.UnitTests/HdrHistogram.UnitTests.csproj -c Release
+dotnet pack HdrHistogram/HdrHistogram.csproj -c Release --no-build
+```
+
+After `dotnet pack`, confirm the `.nupkg` contains `lib/net8.0/`, `lib/net9.0/`, `lib/net10.0/`, and `lib/netstandard2.0/` folders.
+
+## Risks and Open Questions
+
+- **net10.0 SDK availability**: The devcontainer currently uses a .NET 9.0 base image with an additional 8.0 runtime.
+  A .NET 10.0 SDK must be available in CI (GitHub Actions `setup-dotnet@v4` supports `10.0.x`) and locally.
+  If the devcontainer does not have .NET 10 installed, local builds targeting `net10.0` will fail.
+  The CI step is the authoritative build environment; local failure is acceptable during transition.
+
+- **BenchmarkDotNet compatibility**: `BenchmarkDotNet` version `0.13.12` (currently referenced) must support net10.0.
+  If it does not, the version pin may need updating.
+  This should be verified during implementation by attempting a Release build of the benchmarking project.
+
+- **Examples project**: The issue specifies updating to `net10.0` only (single target, not multi-target).
+  This is intentional — examples are a developer-facing runnable tool, not a shipped library.
+
+- **Spec update**: `spec/tech-standards/build-system.md` still documents AppVeyor CI.
+  That section was already outdated before this issue.
+  Only the target framework tables and CI setup sections need updating here; AppVeyor cleanup is out of scope.