feat(harden): alien-emit rethink — CFG dispatch lite + native MBA pads

[undivisible]

Summary

• Deepens harden/anti-decomp beyond merged feat: harden + lean anti-decomp emit profiles #296 with lite CFG dispatch (while _pc < N state machine for eligible straight-line bodies; skips loops/try/match/throw; avoids Break because native lower treats it as a no-op).
• Native alien shapes: rotating prologue variants, MBA constant materialization (^m, ±m, dual-mask), rotating junk pads wired before calls (junk_pad was previously unused).
• Updates docs/emit-profiles.md honesty; refreshes objdump/nm smoke metrics (harden object 2689→3109 bytes). Ghidra headless section retained from prior run (this host lacked GHIDRA_INSTALL_DIR).

Test plan

• cargo fmt (in-cli)
• cargo test --lib harden_ / antidecomp / emit_profile / lower_simple_default_and_harden / core_opt::
• bash scripts/ghidra-antidecomp-smoke.sh (objdump/nm; Ghidra skipped on this host)
• CI: Self-host + JIT conformance + fmt

Do not merge until CI green and parent says so.

---

Note

Medium Risk
Harden-only IR restructuring and codegen shape changes can affect semantics or ABI if dispatch eligibility or no-op pads are wrong; default/lean paths are unchanged but harden output is larger and harder to debug.

Overview
Extends the harden emit profile with lite CFG dispatch on Core IR and rotating native anti-decomp shapes, plus doc/benchmark refresh.

IR (core_opt): After existing harden passes, eligible straight-line function bodies (≥3 stmts, no loops/Break/try/match/throw) are rewritten into a while _pc < N loop with chained if _pc == i states; large bodies are chunked (2 stmts per state when >8 stmts). Dispatcher exits by advancing _pc past N—not via Break, which native lower still treats as a no-op. A unit test asserts _pc + dispatcher loop appear under EmitProfile::Harden.

Native (antidecomp + x86_64_lower): A thread-local tick rotates three prologue variants, four MBA constant-load patterns for weird_materialize_rax, and four junk_pad sequences. junk_pad is now emitted immediately before every call (internal and extern) when harden is active—previously unused at the lowering site.

Docs: emit-profiles.md documents CFG dispatch limits and native rotation; ghidra-antidecomp-smoke.md metrics reflect a larger harden object (2689 → 3109 bytes) and note Ghidra headless was skipped on the regenerating host.

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