fix: avoid unstable duration_constructors so torchft builds on Rust < 1.91

[ajinkyajawale14499]

Summary

src/net.rs calls Duration::from_mins(1). Duration::from_mins/from_hours (the
duration_constructors feature, rust-lang/rust#120301) were unstable until Rust 1.91, so building
torchft on older stable Rust fails with:

error[E0658]: use of unstable library feature `duration_constructors`
  --> src/net.rs:20:36

This was reported on Rust 1.88 (#330). CI did not catch it because every workflow installs
--default-toolchain=stable, which is now >= 1.91.

The issue suggests documenting Rust 1.91 as the minimum. This PR instead removes the need for it:
from_mins(1) is exactly from_secs(60), so switching to the stable constructor lets torchft build
on the older stable Rust the reporter (and many distro/conda/CI environments) actually have, rather
than forcing everyone onto a bleeding-edge toolchain for a trivial constant.

Changes

The API was used at four sites (one in production, three in tests), all converted to the identical
from_secs(...) value:

• src/net.rs — from_mins(1) -> from_secs(60) (HTTP/2 keep-alive interval)
• src/timeout.rs (tests) — from_hours(3)/from_mins(3) -> from_secs(3 * SECONDS_IN_HOUR) /
  from_secs(3 * SECONDS_IN_MINUTE), reusing the module's existing constants so the test mirrors the
  parser it covers
• src/lighthouse.rs (test) — from_hours(10) -> from_secs(10 * 60 * 60)

No behaviour change: each replacement is the same Duration.

Verification

• rustup toolchain install 1.88.0
• On main: cargo +1.88.0 check --tests reproduces E0658 at all four sites.
• With this change: cargo +1.88.0 check --tests compiles clean (build floor dropped to <= 1.88).
• cargo +nightly fmt --check clean; cargo +stable check --tests green.
• grep -rn "from_mins\|from_hours\|from_days" --include='*.rs' returns nothing.

Follow-up

If you would still like the minimum Rust version documented explicitly, I am happy to add a
rust-version field to Cargo.toml plus a README note in a follow-up. Removing the unstable API
first means that floor can stay conservative instead of being pinned to 1.91.

Closes #330

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