feat(xpkg): exports schema + predicate-driven elfpatch

[Sunrisepeak]

Summary

Adds xpm.<platform>.exports.runtime.{loader, libdirs, abi} schema field and rewrites elfpatch.lua to be predicate-driven post-install instead of relying on consumers to call elfpatch.auto({...}) in install hooks.

Detailed design + rationale: openxlings/xlings docs/plans/2026-05-02-elfpatch-exports-design.md (incoming PR).

Schema additions

xpm = {
    linux = {
        deps = { runtime = {...}, build = {...} },
        exports = {                                       -- new field
            runtime = {
                loader  = "lib64/ld-linux-x86-64.so.2",  -- libc family
                libdirs = { "lib64", "lib" },            -- non-default only
                abi     = "linux-x86_64-glibc",          -- multi-libc only
            },
        },
        ...
    },
}

All sub-fields optional. Providers only declare non-default behavior.

elfpatch.lua

New consumer-facing API:

• elfpatch.set(opts) — override predicate-driven auto with explicit params
• elfpatch.skip() — opt out of auto-patch entirely

Old elfpatch.auto({...}) retained as deprecation alias mapping to elfpatch.set({...}). Drop after 2026-11. Debug-level deprecation log on first call.

_apply() decision tree:

1. user_skip → return
2. user_override → use hook-given opts
3. self_exports.loader set → patch with own loader (glibc self-patch)
4. exactly 1 runtime-dep with exports.loader → use it
5. ≥ 2 runtime-deps with exports.loader → fail-fast, require interp_from
6. otherwise → no-op

Plus: e_machine cross-arch protection, static-binary skip, runtime_deps_list (not union) for RPATH closure → fixes 0.0.32 build_dep pollution.

_RUNTIME extensions

• runtime_deps_list / build_deps_list (split, post-#249)
• deps_exports lookup { [dep_spec] = { loader, libdirs, abi } }
• self_exports

Test plan

• LoaderTest: 11/11 pass (3 new fixtures: full / loader-only / no-exports)
• After merge: tag v0.0.33; downstream registers in mcpplibs-index; xlings PR bumps add_requires

Cross-repo

This PR alone is non-breaking — no consumer reads the new field yet, deprecation alias preserves old hook API. Real consumer impact lands after:

1. Merge + tag v0.0.33
2. mcpplibs-index registers 0.0.33
3. openxlings/xlings PR bumps add_requires (drafted)
4. d2learn/xim-pkgindex glibc/binutils pilot (drafted)