chore(publish-prep): opt the 5 publishable lib crates into docs.rs cfg-badge rendering

[githubrobbi]

What

Adds #![cfg_attr(docsrs, feature(doc_cfg))] to the lib.rs of every publishable library crate so that docs.rs renders cfg-availability badges on cfg-gated public items.

The 5 lib crates touched:

Crate	Insertion site	Existing #![…] neighbours
uffs-time	crates/uffs-time/src/lib.rs	grouped with #![no_std]
uffs-text	crates/uffs-text/src/lib.rs	standalone (no other #![…])
uffs-mft	crates/uffs-mft/src/lib.rs	grouped with the existing #![warn(…)] cluster
uffs-client	crates/uffs-client/src/lib.rs	standalone
uffs-mcp	crates/uffs-mcp/src/lib.rs	standalone

uffs-cli is binary-only (no lib.rs) — no directive needed.

Why

Each publishable crate already declares the docs.rs rendering hint in its manifest:

[package.metadata.docs.rs]
all-features = true
rustdoc-args = ["--cfg", "docsrs"]

--cfg docsrs only takes effect when the crate root opts in. With this attribute, items behind #[cfg(feature = "async")], #[cfg(windows)], #[cfg(target_os = "…")], etc. render with their cfg-availability badge on docs.rs. Without it, those badges silently don't appear.

Local cargo doc does not pass --cfg docsrs, so the cfg_attr wrapper bypasses the directive entirely — the nightly-only doc_cfg feature is never exercised in normal workflows.

Why doc_cfg, not doc_auto_cfg

The earlier-typed doc_auto_cfg got rejected by rustc:

error[E0557]: feature has been removed
  --> crates/uffs-text/src/lib.rs:38:29
   |
38 | #![cfg_attr(docsrs, feature(doc_auto_cfg))]
   |                             ^^^^^^^^^^^^ feature has been removed
   |
   = note: removed in 1.92.0; see <https://github.com/rust-lang/rust/pull/138907> for more information
   = note: merged into `doc_cfg`

Rust 1.92 (rust-lang/rust#138907) merged doc_auto_cfg into doc_cfg. The unified doc_cfg feature provides both the explicit #[doc(cfg(…))] attribute and the automatic cfg-badge inference behaviour that doc_auto_cfg used to offer alone. Our pinned nightly (nightly-2026-05-14 per rust-toolchain.toml, rustc 1.97-nightly) is well past the merge.

Verification

Locally on macOS Apple Silicon, all of these pass:

cargo check -p uffs-time -p uffs-text                        # OK
RUSTDOCFLAGS="--cfg docsrs" cargo doc -p uffs-time -p uffs-text -p uffs-mft -p uffs-client -p uffs-mcp --no-deps   # OK
cargo fmt --check                                            # OK
cargo clippy -p uffs-time -p uffs-text -- -D warnings        # OK

The pre-push lint-pre-push gate (smoke + lint-ci-windows) also passed before push.

Adherence to the 5 user-mandated rules

1. No suppression hacks — the directive is gated behind cfg(docsrs) which is the canonical docs.rs feature-opt-in pattern (used by tokio, serde, tracing, …); it does not suppress any lint, warning, or test.
2. Surgical fix — 36 lines added across 5 files, each a single #![cfg_attr(…)] line plus a justifying comment block.
3. Preserve behaviour — no public API changes. Local builds (no --cfg docsrs) bypass the directive entirely; only docs.rs rendering is affected.
4. Tests — no existing tests touched; no new tests added because the directive has zero runtime / type-system surface (it's a rustdoc-only opt-in).
5. Documented & atomic — single commit, conventional-commit subject, detailed body explaining the why and the doc_auto_cfg → doc_cfg migration.

Scope context

Part of the crates.io publish-readiness umbrella tracked in #241, specifically gap #2 (the doc_auto_cfg directive item). One more PR-B remains in that umbrella (per-crate keywords + categories allow-list + apply) before the actionable-now backlog is closed.

Test plan

• cargo check on the 2 polars-free crates
• RUSTDOCFLAGS="--cfg docsrs" cargo doc --no-deps on all 5 lib crates
• cargo fmt --check
• cargo clippy -- -D warnings on the 2 polars-free crates
• lint-pre-push (smoke + lint-ci-windows) passes pre-push

Refs: #241