feat(0.4.48): owner-anchored shim dispatch (owner -> env -> default)

[Sunrisepeak]

What

A shim is an artifact of exactly one home. Dispatch now resolves against the home that owns the shim file first, instead of trusting ambient XLINGS_HOME:

shim dispatch home: owner home (exe-anchored) -> env XLINGS_HOME (deprecated, warns) -> ~/.xlings

• XLINGS_HOME keeps exactly one meaning: the management target of the xlings CLI (install/update). CLI home resolution is untouched.
• XLINGS_SHIM_ANCHOR=legacy restores pre-0.4.48 env-first dispatch (release insurance, one soak cycle).
• Design + impact analysis: .agents/docs/2026-06-04-shim-owner-anchoring-design.md (updated to as-landed state in this PR).

Why

Any process tree that redirects XLINGS_HOME (e.g. mcpp's vendored registry sandbox) silently broke every shim-resolved tool on PATH — git, python, node… The git shim failing inside mcpp's environment deadlocked package-index sync (index needs git, git bootstrap needs index) and left Windows users with mcpp build / mcpp self init --force permanently failing while their terminal git --version worked fine. Reproduced end-to-end in mcpp-community/mcpp#114; probe:

XLINGS_HOME=~/.xlings        git --version -> git version 2.51.0.windows.2
XLINGS_HOME=~/.mcpp/registry git --version -> [error] xlings: 'git' is not installed

Changes

• src/core/xvm/shim.cppm — is_home_root (structural signature: .xlings.json + bin/xlings + subos/), resolve_owner_home (innermost-home upward walk), home_knows_program (lightweight versions-DB probe, no Config involvement), resolve_dispatch_home (the chain, legacy mode, deprecation/ambiguity warnings)
• src/core/config.cppm — Config::override_home() pre-init hook; ctor honors it ahead of env/self-contained; version 0.4.48
• src/main.cpp — run the chain for shim invocations before first Config use
• src/core/xself/doctor.cppm — check 2.6 shim anchor (warning-only; scoped to binDirs physically inside the home so project-subos binDirs don't false-positive)
• tests/unit/test_shim_anchor.cpp — 14 tests: home-root signature, owner walk (incl. nested innermost + orphan), DB probe, chain ordering
• tests/e2e/shim_owner_anchoring_test.sh — owner-wins + warning / deprecated borrowing + warning / legacy mode / redirected-env (mcpp) regression; registered in linux + macos CI

Behavior changes

Only one: when both the owner home and the env home have the tool, the owner now wins (previously env won) — with a visible ambiguity warning. Borrowing (tool only in env home) still works, with a deprecation warning. Everything else is unchanged or was previously a hard error.

Verification

• mcpp build clean; tests/unit pass (pre-existing local-env Extract.* failures reproduce identically on clean main, unrelated)
• e2e script passes all 4 scenarios locally
• Follow-up after release: re-run [temp][DO NOT MERGE] Windows no-git default-user flow repro CI mcpp-community/mcpp#114 CI against the new release to confirm the Windows user-flow repro is fixed