build: support zig 0.16

[banteg]

Summary

• require Zig 0.16.0 in build.zig.zon
• mark the struct field slice as comptime for the inline for helper
• remove an unused tagged-union payload capture rejected by Zig 0.16

Tests

• ./check.sh --ci

Summary by CodeRabbit

• Chores

  • Updated minimum Zig toolchain requirement to version 0.16.0.

• Refactor

  • Internal code improvements and optimizations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 4440eebf-4579-4012-93b4-b58822a5ed8d

📥 Commits

Reviewing files that changed from the base of the PR and between ca0f153 and 7cd4568.

📒 Files selected for processing (3)

• build.zig.zon
• src/struct.zig
• src/union.zig

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📝 Walkthrough

Walkthrough

This pull request bumps the Zig toolchain requirement from 0.15.1 to 0.16.0 in the build manifest. It also adds a comptime qualifier to the fields parameter in countUsedStructFields and removes an unused variable binding in the union field unpacking switch statement.

Changes

Cohort / File(s)	Summary
Toolchain Version Update build.zig.zon	Incremented minimum_zig_version from "0.15.1" to "0.16.0" to reflect updated toolchain requirement.
Struct Field Counting src/struct.zig	Added comptime qualifier to the fields parameter of countUsedStructFields, requiring struct field metadata to be known at compile-time.
Union Field Unpacking src/union.zig	Removed unused bound variable from switch-arm pattern in unpackUnionAsTagged for the .field_name_prefix case, changing |_| to no binding.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 Hopping through our Zig-zag code so neat,
We've bumped the version—quite a treat!
With comptime fields and bindings cleared away,
Our structs and unions dance and play! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title 'build: support zig 0.16' accurately reflects the main objective: updating the project to support Zig 0.16 by bumping the minimum version requirement and making necessary code adjustments.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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