docs: add CLAUDE.md (thin pointer + stack specifics)

CLAUDE.md

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+# CLAUDE.md — Homelab.Stacks.BuildLab
+
+Guidance for Claude Code working in this repo.
+
+## What this is
+
+A **stack submodule** of [`Chrison-Homelab/Homelab`](https://github.com/Chrison-Homelab/Homelab),
+mounted there at `stacks/BuildLab` (meta-repo model, ADR-0008). This repo holds the stack's shapes
+plus the Windows provisioning assets; the **converge/validate engine lives in the superproject**
+and is the only thing that applies them.
+
+> **Read the superproject's [`CLAUDE.md`](https://github.com/Chrison-Homelab/Homelab/blob/main/CLAUDE.md) first.**
+> It carries the rules that apply here and are *not* repeated in this file: the PR-only git
+> workflow + merge strategy (ADR-0010), the worktree rule for parallel sessions, the shared
+> external-account guardrails, and `secrets.env` / Bitwarden Secrets Manager handling.
+
+## This stack
+
+**Windows dev/build VMs** for testing the Fallout build project across multiple Visual Studio
+toolchains.
+
+- **VMID block:** `1100–1199` (enforced by `stack.yaml`'s `ctidRange`)
+- **Default node:** `desktop-01` — strongest CPU (Ryzen 5 3600) and the proven Win11 recipe
+- **Current member:** `buildvm` — **VMID 1100**, `kind: VM`
+
+**`kind: VM`, not LXC.** These are provisioned by the **ProxmoxSharp write path** (the same path
+the Gaming stack uses), *not* the community-scripts `ct/<app>.sh` path that the LXC stacks use.
+Members set `vmid` explicitly — omission is an error.
+
+`spec.defaults` in `stack.yaml` reuses the LXC spec shape (per the schema), so only fields common
+to both (`node`/`timezone`/`tags`) belong there. VM-specific fields (`machine`/`bios`/`cpu`) stay
+in the member shape — same convention as the Gaming stack.
+
+## Working here
+
+Converge runs **from the superproject**, pointed at this directory:
+
+```bash
+# in the superproject
+dotnet run --project Infrastructure/engine -- validate stacks/BuildLab
+dotnet run --project Infrastructure/engine -- converge stacks/BuildLab          # dry run
+dotnet run --project Infrastructure/engine -- converge stacks/BuildLab --apply
+```
+
+## Gotchas specific to this stack
+
+Windows unattended provisioning is unforgiving; these all cost real debugging time:
+
+- **The autounattend ISO must be built WITHOUT Joliet.** Joliet mangles long filenames, and the
+  provisioner then can't find its own files.
+- **Provisioner scripts must be written as UTF-8 without CP1252 smart characters.** An em-dash in a
+  PowerShell provisioner silently corrupted execution. Keep ASCII in anything that ends up on the
+  Windows guest.
+- **VS 2026 uses channel `18/stable`, not `18/release`.** The wrong channel installs nothing while
+  appearing to succeed.
+- **Stale node artifacts win over new ones.** If a rebuild seems to have no effect, check for an
+  older ISO/answer file still sitting on the node.
+- **Guest configuration goes through `qm guest exec`** (the agent is on the VM), not by fetching
+  raw URLs — this is a private repo, so `irm` of a raw GitHub URL will not work.
+
+All six VS toolchains (2019/2022/2026 × Community + BuildTools) plus .NET 10 install unattended as
+of the last verified build.