refactor(zero-c): consolidate duplicated dynamic-array grow helpers (#390)

Summary

Fixes #390.

The same dynamic-array grow helper was duplicated across four files in native/zero-c/src:

• ir_grow_items (ir.c), lower_grow_items (program_graph_lower.c), and checker_grow_items (checker.c) shared an identical body.
• ir_grow_tracked_items (the variant that also accounts ir->mir_bytes) was defined verbatim in both ir.c and program_graph_mir.c.

As the issue notes, the duplicated mir_bytes accounting can silently diverge if one copy is edited without the others.

Change

Hoist both helpers into include/zero.h as shared static inline functions (z_grow_items and ir_grow_tracked_items), placed right after the IrProgram definition so the tracked variant can reference ir->mir_bytes. The five local definitions are removed and their call sites repointed at the shared helpers.

Pure refactor — the helper bodies are byte-for-byte the originals, so there is no behavior change. Net +61 / -81 lines across 5 files.

Validation

• make -C native/zero-c builds clean under -Wall -Wextra -Wpedantic.
• Host-target end-to-end works: zero build --emit exe --target linux-x64 on conformance/run/pass/hello.graph produces an exe that runs and prints the expected output.
• All conformance/check/pass/*.graph fixtures pass zero check (this is the path that exercises the modified checker.c and ir.c lowering).

Transparency per AGENTS.md: I developed this on Linux (Ubuntu via WSL) where the linux-x64 host target is fully runnable, but pnpm run agent:checks could not complete because the linux-musl-x64 build steps require a musl-capable C compiler that isn't installed in my environment. That step fails identically on a clean checkout (BLD003: cross target 'linux-musl-x64' requires a target-capable C compiler ... install the bundled-toolchain default), so it is pre-existing and unrelated to this change. CI (with the musl toolchain) should exercise the full matrix.