InlineZig burndown: registry-backed template bridge

[cuzzo]

Non-low-risk InlineZig cluster from the opaque-Zig hardening pass.

Sites:

• src/mir/mir_lowering.rb:2850 static_call
• src/mir/mir_lowering.rb:3201 builtin_*
• src/mir/lowering/functions.rb:1731 intrinsic
• src/mir/lowering/variables.rb:1124 index_set

Risk:

• These are generic Zig template-substitution bridges. Some entries allocate, mutate receivers, transfer ownership, or take allocator placeholders. They are guarded by registry metadata, but still hide operation structure behind Zig text.

Acceptance:

• Replace with structural MIR or narrower typed operation nodes where doing so reduces or preserves overall complexity.
• Any remaining InlineZig must be documented as low-risk or lower-complexity than the structural alternative.
• Preserve MIRChecker ownership visibility and existing BC paths.

[cuzzo]

Current status after the pushed InlineZig burndown commits: registry-backed static_call, builtin_*, intrinsic, and index_set remain. These are not ad-hoc snippets; they route through stdlib/intrinsic metadata and ownership contracts. Removing them elegantly requires migrating registry entries to typed structural call/index-store nodes rather than expanding per-op lowering branches.

Decision: keep #96 open. I did not replace these with a larger switch because that would increase complexity and duplicate the registry.