Fix Shen-CL kernel bootstrap

[pyrex41]

Summary

• make the default fetch target assemble the supported Shen 41.2 hybrid kernel
• keep the unmodified community archive available as fetch-community
• use the supported kernel path in CI and release workflows
• clarify the bootstrap workflow and kernel provenance in the README

Root cause

The community ShenOSKernel 41.2 archive initializes *property-vector* in init.kl, while this Shen-CL branch intentionally builds against Mark Tarver’s refreshed kernel layout and omits that legacy initialization path. A fresh make fetch therefore produced an incompatible kernel and failed during SBCL bootstrap with an unbound *property-vector*.

Validation

• fresh make fetch
• make precompile SHEN=/Users/reuben/projects/shen-go/.bin/shen-go
• make build-sbcl
• 134 upstream kernel tests passed
• 136 Shen-CL port assertions passed
• 16 CLI parity checks passed
• bin/sbcl/shen --version reports Shen 41.2

Closes #15