The async pin-extraction helpers had two divergent shapes:

- mmap_file/{tokio,smol}_impl.rs: `extract_pin_or_err(File) -> Result<File, (AsyncFile, Error)>` (with file recovery on dup failure)
- disk/{tokio,smol}_impl.rs: `extract_inode_pin(File) -> io::Result<File>` (no recovery)

Same purpose, different return types. Consolidate to the
recovery-aware shape across both module pairs (and a shared name,
`extract_pin_or_err`). Tokio's helper still always succeeds (Ok via
into_std); smol's helper returns the file on dup failure for caller
recovery. The disk macro currently discards the recovered file
(trait method's `Result<()>` shape can't return `Self`); the
recovery-aware variant is now ready for a future `try_drop_remove`
inherent method (see #20).

Towards #17.

The raw `AsyncDiskMmapFileMut::drop_remove(self)` consumed `self`
unconditionally — including on the only practical recoverable
failure mode, smol's `fcntl_dupfd_cloexec` returning `EMFILE` while
extracting the inode pin. Caller had no way to retry.

New inherent method:

\`\`\`rust
impl AsyncDiskMmapFileMut {
    pub async fn try_drop_remove(self)
        -> Result<(), DropRemoveError<Self>>;
}
\`\`\`

`DropRemoveError<T>` (in `fmmap::error`) has two variants:

- `Recoverable(T, Error)` — failed before any destructive op (only
  smol-EMFILE in practice). `Self` is preserved so the caller can
  retry once fd pressure subsides.
- `Terminal(Error)` — failed after destructive op (probe / unlink /
  parent fsync). `Self` is gone; treat like `Result<(), Error>`.

The trait method `AsyncMmapFileMutExt::drop_remove` now delegates to
`try_drop_remove` and discards the recovered value via
`DropRemoveError::into_error` for users who don't need recovery.
The trait signature is unchanged (`Result<()>`).

Tokio's `into_std()` allocates no fd, so the `Recoverable` variant
is unreachable on tokio; this is purely additive there.

Closes #20.

The `DropRemoveError<T>` type added by 82db381 implements
`core::error::Error`, which was stabilized in Rust 1.81. The
crate's previous MSRV was 1.75, so users on the documented
MSRV would hit E0658.

Bump everywhere:
- `fmmap/Cargo.toml`: rust-version = "1.81.0"
- `README.md`: "Rust **1.81** or later"
- `fmmap/src/disk.rs`: lock-API rationale comment
- `.github/workflows/ci.yml`: minrust job pins to 1.81.0

minrust-smol stays at 1.85 (smol's transitive dependencies require
that floor independently of the crate-level MSRV).

Verified `cargo +1.81.0 check` clean for sync and tokio features.