fix(install.ps1): probe WSL2 networking before delegating to bootstrap (closes #1006)

[joelteply]

Summary

Add a 5s curl probe from inside WSL2 to raw.githubusercontent.com BEFORE delegating to bootstrap.sh. When WSL2 networking is broken (vEthernet / HNS corruption is common on Win10/11 after sleep cycles or driver updates), the curl inside the bootstrap step takes 30+ seconds to time out with a cryptic error — and the user has zero signal that the issue is environmental, not continuum-related.

Caught live 2026-05-02 by continuum-b69f during Carl-OOTB Windows testing (issue #1006). After PR #1005 fixed the WSL detection, install.ps1 delegated into bootstrap successfully — and the WSL-side curl just hung. The user has no way to distinguish "install is broken" from "my box's WSL is broken."

Fix

5s probe upfront. On failure, surface explicit remediation:

1. wsl --shutdown (forces VM restart, often heals NAT)
2. (as admin) Restart-Service hns -Force (reset Host Networking Service)
3. Reboot Windows
4. Edit %USERPROFILE%\.wslconfig → [wsl2] networkingMode=NAT

• Re-run install command

Same pattern as install.sh's friendly-failure phase traps (#977 family): fail loudly + tell the user exactly what to try next, instead of dying silent or with a 30s mystery timeout.

Test plan

• CI green (Linux/Mac unaffected; this is install.ps1-only)
• Live: b69f's box (Carl-OOTB Windows: bootstrap.sh fetch fails — WSL2 cannot reach raw.githubusercontent.com (Phase 2 blocker) #1006 had WSL2 NAT broken — perfect natural test) — re-running install.ps1 should now print the probe-failure remediation message instantly instead of hanging in bootstrap.sh
• b69f's reboot: WSL2 net healed, Phases 1-3 all PASS in their next run, confirming the bootstrap delegate works when probe would have passed

Closes

• Carl-OOTB Windows: bootstrap.sh fetch fails — WSL2 cannot reach raw.githubusercontent.com (Phase 2 blocker) #1006 — surfaces the WSL2 NAT failure mode immediately rather than burying it in a 30s curl hang inside bootstrap.sh

Related

• Depends on fix(install.ps1): WSL detection — strip null bytes from UTF-16 wsl --list output #1005 (UTF-16 wsl-list fix) for Phase 1 to even reach this delegation point — order matters for Carl-OOTB Windows
• Stacks naturally with fix(git_bridge): strip inherited git-context env in run_git (kills pre-push hook race) #1009 (pre-push hook fix) which unblocked this PR's path through the hook

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