fix(ci): pin time to 0.3.51 to unbreak cookie 0.18.1 build

[sanity]

Problem

CI's build job is red for every PR and push as of 2026-06-30:

error[E0061]: this method takes 2 arguments but 1 argument was supplied
   --> cookie-0.18.1/src/parse.rs:226:27
   --> time-0.3.52/src/parsing/parsable.rs:62:12
error: could not compile `cookie` (lib) due to 1 previous error

This repo commits no Cargo.lock and CI does not pass --locked, so every run re-resolves dependencies from crates.io. time 0.3.52 (published today) changed the Parsable::parse signature; cookie 0.18.1 — a dev-dependency of room-contract via the freenet crate, compiled by cargo test --package room-contract --features net — still calls it with the old arity. Nothing in the source changed; the dependency graph drifted.

Approach

time can't just be held below the break — serde_with 3.21.0 (via freenet-stdlib 0.6.1) requires time = "~0.3.47". The compatible window is 0.3.47–0.3.51. Pin to 0.3.51 (newest that satisfies the floor and stays below the break) using the same cargo update --precise mechanism already used for constant_time_eq, in both the build and ui-playwright-tests jobs.

Testing

Reproduced and verified locally:

• cargo test --package room-contract --features net --no-run with time 0.3.52 → fails on cookie (E0061).
• After cargo update -p time --precise 0.3.51 → cookie and the room-contract test build compile cleanly.

CI on this PR is the end-to-end confirmation.

Follow-up

This class of break recurs because CI re-resolves on every run. The durable fix is to commit a Cargo.lock (or build with --locked) so CI stops drifting with each upstream publish. Pinning here is the minimal unblock; the lockfile question is worth a separate discussion. The time pin should be dropped once cookie ships a time 0.3.52-compatible release.

[AI-assisted - Claude]