fix: payload discovery must skip delegating-package husks (#120)

[Sunrisepeak]

Fixes #120 (ci-linux e2e 29_toolchain_partial_versions.sh / 31_transitive_deps.sh failing on main).

Root cause (forensics)

1. Why it never failed before: green runs restored a warm 5.2 GB ~/.mcpp cache holding an anciently-installed, complete toolchain — the cold first-run install path was never exercised. The old cache got evicted; the next run's bare restore prefix mcpp-sandbox-Linux- then matched release.yml's mcpp-sandbox-Linux-release-… cache (different sandbox flavor), and the run finally hit the true cold path.
2. The actual bug: xim:linux-headers is a delegating package — the payload installs under scode-x-linux-headers/…, while xim-x-linux-headers/<v>/ is left as a metadata-only husk (.xim-installed + .xpkg.lua). find_sibling_package counted .xpkg.lua as content, returned the husk, and probe_payload_paths silently dropped the kernel-header include dir → glibc's bits/local_lim.h fails at #include <linux/limits.h> in the std-module precompile.
3. find_home_tool (the fallback) was worse: hardcoded xim-x- prefix, no content validation — it also returned the husk.

Fix

• Payload qualification helper shared by both finders: dot-prefixed entries are install metadata, not content; an optional requiredRelPath must exist inside the version dir (probe passes include/linux/limits.h / include/features.h).
• find_home_tool now scans all index prefixes (xim-x-, scode-x-, …) with the same rules.
• Probe logs a verbose warning instead of silently proceeding without kernel headers.
• CI cache hygiene: -ci- cache lineage for ci-linux/ci-windows, disjoint from release's -release-; release no longer falls back to the bare prefix either.

Verification

• 4 new unit tests (XlingsSiblingPackage.*, XlingsHomeTool.*): husk-only → no match; husk + scode payload → scode payload wins; contentful-but-wrong candidate rejected via requiredRelPath; non-xim prefix found via home lookup. Full suite 18/18.
• Local e2e 29/31 pass (host environment masks the cold path, so the discriminating coverage is the unit layer).
• The cache key change makes this PR's ci-linux run fully cold — it exercises the exact first-run install path that broke on main, so a green run here is the end-to-end proof.