build(toolchain): bump Zig to dev.2085 — @hasDecl is now pub-only

[dbgeek]

What broke

nix flake update moved the zig-overlay pin 0.17.0-dev.1786+75044cb04 → 0.17.0-dev.2085+5e36170b5, and every zig build — including zig build install-agent — died with 26 comptime errors:

src/grants.zig:186:13: error: type 'grants.FakeProbes' is not Grant Probes: missing method 'micGranted'
src/session.zig:382:13: error: type 'session.FakeTransport' is not a Transport: missing method 'init'
src/local_backend.zig:94:13: error: type 'local_backend.FakeHelper' is not a Helper: missing method 'isReady'
...

Why

Upstream Zig narrowed @hasDecl to report only pub declarations. This is a deliberate, documented language change — the langref shipped inside each compiler says so:

Zig	langref wording	@hasDecl(Priv,"m") from Priv's own file
dev.1786 (before)	"has a declaration matching name"	true
dev.2085 (after)	"has a public declaration matching name"	false

Verified by differential probe on both toolchains, not inferred.

Five files define comptime contract assertions — assertProbes, assertHelper, assertTransport, and assertDeps (undo + insertion_runner) — that check a type's method surface with @hasDecl. Every production adapter already declares those methods pub. Every test fake declared them bare fn, which was legal only because the assertion lives in the same file as the fake, and same-file visibility used to satisfy @hasDecl. Under the new rule the fakes stop matching their own contracts and each assertion fires @compileError.

A tell that the seam was always fragile: std.meta.hasFn lives in a different file and so already returned false for private decls on dev.1786. These hand-rolled @hasDecl checks were leaning on a visibility rule std's own equivalent never offered.

The fix

pub on the contract methods of FakeProbes, FakeDeps ×2, FakeHelper, and FakeTransport, plus FakeTransport's Reader handle. 46 lines, pub and nothing else — no runtime behaviour change, no production code touched. Test-only helpers like FakeProbes.requested stay private, so the pub marker now says precisely which members are the contract.

Verified

Command	Result
zig build	exit 0
zig build test	exit 0, all suites OK
zig build install-agent	exit 0, signed + installed pair-1789068005-16478

⚠️ This bump is deliberately partial

docs/toolchain.md requires the compiler and websocket.zig to move as one atomic change. This PR does step 1 and the fallout only. Outstanding:

• Step 2 — bump websocket.zig. docs/research/websocket-zig-bump.md (untracked on main at time of writing) recommends folding the pin bump to bfe761959b05030eaf4943fcf0fd5ecd1daca68a into "the next compiler bump" — which is this one. Not done here. Note that research proved the current pin 4b475a8 compiles under dev.1786; it has not been re-proved under dev.2085.
• Step 3 — raise the floor. minimum_zig_version is 0.17.0-dev.1267+300116b02 in all six build.zig.zon files. This drift is pre-existing (it was already two nightlies stale before this PR), not introduced here. docs/toolchain.md's "currently pinned pair" table is stale for the same reason.
• Step 4 — the live re-prove. No wss round-trip against wss://api.openai.com/v1/realtime was run. zig build test does not exercise the live transport.

Happy to do any of these in follow-ups; they were left out to keep this PR to the diagnosed break.

One thing worth a second look

src/local_backend.zig:194:

if (comptime !@hasDecl(Helper, "usesModel")) return true;

This is the only optional @hasDecl in the tree — it degrades silently rather than erroring. Under the new semantics it would have quietly reported "installation still valid" forever for any Helper with a non-pub usesModel, instead of failing to compile. It's saved only because usesModel is also in assertHelper's required list, so the hard assertion fires first. That redundancy is load-bearing, and the line is now dead weight — left alone here, but it's the shape of bug this bump could have caused instead of a clean compile failure.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JGU4Zq5h2Hf7pTXDuQNa3w