exec: PE32+ loader, versioned ABI, first external binaries (v0.6.0)

rsapi: the kernel<->binary ABI crate (no_std, no deps).  Versioned
Exports table (magic + version header), EntryPoint = extern "C" on
x86_64-unknown-uefi (Win64: RCX/RDX/R8), Line/Arg/StreamResult types.
Binaries link only this crate.

rsnix::load: PE32+ parser — DOS/COFF/optional headers, section copy,
BSS zero-fill, .reloc base relocations (DIR64) via rva_to_file mapping
(the reloc directory is an RVA, not a file offset).  Images are
allocated at LOAD_BASE=192MiB inside the page allocator's conventional
span and identity-mapped user+writable+nx.

rsnix::exec: resolution name -> /disk/<UID>/bin/<kind>/<name>.efi per
mounted volume; spawn reserves a sched slot, stores a per-task ExecCtx
(exports + argv block + image) before launch_at installs the frame —
no adopt race.  Trampoline calls entry(&Exports, argc, argv), posts
TaskMsg::User(exit_code) to the shell.  abort_current notifies the
shell with u64::MAX when an exec'd binary dies, so a faulting command
cannot leave the shell blocked forever.

sched: reserve_slot/launch_at split out of spawn; task_exit made pub.

shell: exec_external runs before non-frozen built-ins (ADR 0006
resolution rule); frozen core (help/clear/echo/halt/poweroff/exit/
rescan) is exempt.  Exports v1 wires put_line to the terminal and
get_line to EOF; stream injection lands in Phase 3.  ^C while waiting
kills the command task.

bins/{hello,boom}: gate binaries.  hello prints through the exports
table; boom faults on 0x400000000000 to prove containment.  The image
entry point IS the ABI entry (efi_main carries the Exports signature).

Version 0.6.0: exec exists end-to-end.

Gates verified in QEMU: 'hello' loads from \\bin\\cli\\hello.efi, calls
put_line across the ABI, exits 0, shell unblocks; 'boom' faults, task
dies gracefully with full report, shell survives, jobs shows the corpse;
built-in fallback works when the file is absent; test-disk PASS.

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