ci: define and enforce the Zig toolchain policy

TOOLCHAIN.mdadded+23-0

Changes to TOOLCHAIN.md

		@@ -0,0 +1,23 @@
	1	+# Zig toolchain policy
	2	+
	3	+The Zig Test Framework supports the exact Zig development build recorded in
	4	+`.zig-version`. Required CI jobs use that version so pull requests are tested
	5	+against a reproducible toolchain.
	6	+
	7	+Because the project follows Zig development builds, CI also runs a scheduled
	8	+compatibility check against the latest `0.17.0-dev` build. That check is
	9	+informational: an upstream API change should produce an intentional toolchain
	10	+update rather than unexpectedly blocking unrelated pull requests.
	11	+
	12	+## Updating Zig
	13	+
	14	+1. Replace the version in `.zig-version` with the full output of `zig version`.
	15	+2. Run `zig fmt --check .`.
	16	+3. Run `zig build`, `zig build test`, and `zig build examples`.
	17	+4. Run `zig build-lib -femit-docs src/lib.zig` and remove the generated local
	18	+ artifacts after verifying the command succeeds.
	19	+5. Summarize required compatibility changes in the pull request and, for a
	20	+ release, in the changelog.
	21	+
	22	+The compatibility job can also be run manually from GitHub Actions before
	23	+updating the pinned version.