docs(rfcs): correct 0001 toolchain from P0 findings

The Static Linux SDK (musl) cannot emit dylibs and --static-swift-stdlib
is a no-op for shared-library products: Linux artifacts become
$ORIGIN-rpath runtime bundles built natively on Linux runners. Pin Swift
6.3.x, settle D5 (artifacts-in-release, embedded in compiled binaries),
and date the P0-research completion in the decision log.