* feat: support multi-instance library mode via runtime symbol localization
- Add the abi.localize_runtime profile field: the archive build combines the
program object with exactly the runtime/vendor members it reaches and
demotes every external definition except the profile-declared symbols to a
local symbol (darwin: one ld -r pass with -exported_symbols_list; linux:
ld -r then objcopy --keep-global-symbols)
- N archives built under pairwise-distinct prefixes now link into one process
with no symbol collisions and no shared mutable runtime state: each
instance owns a private copy of the allocator, collector, result arena, and
panic sink, so sinks register per instance and a trap poisons only the
instance it fired in
- Document the embedder contract beside the profile spec: one thread per
instance (an instance is never entered from two threads), and values cross
instances only through the embedder's own byte/record marshalling
- Refuse cross-target localized builds with SC3002 (the step runs the host
toolchain's ld/objcopy over host-format objects); absent or false keeps the
classic single-archive artifact byte-for-byte
- Add the two-instance acceptance probe (two archives, two embedder threads,
independent collects, a trap delivered to its own sink exactly once while
the other instance keeps answering) plus symbol-exactness, profile-shape,
and target-posture suites, and schedule the artifact contracts on the gate
host
* fix: tighten runtime localization validation
* fix: harden localized library publication
