feat: add IsReadable and IsWritable

[christiangda]

What

Two predicates for the question fsx cannot currently answer: not what a path is, but what you may do with it.

fsx.IsReadable(path)   // regular file, owner-read bit set
fsx.IsWritable(path)   // regular file, owner-write bit set

Why

The motivating case is a tool that validates a path it will later rewrite — a configuration file, a lock file — and would rather refuse at validation time than fail halfway through a write. Exists and IsFile both pass a read-only file happily, so a validator built from them reports success and the write fails later, in a worse place.

Found while adopting fsx in aizon-shared/machete, which needed exactly this for its --config.file check and had to keep a local helper for it. Everything else it needed, fsx already had — including HasExtension, which is better than the hardcoded suffix check it replaced.

Mode bits, not effective access — and the docs say so

Both inspect the file mode, not the calling process's effective access. That is the portable answer the standard library can give: os.Stat exposes mode bits on every platform, while asking "can I open this?" requires access(2) or an attempted open, neither available portably without cgo or a build tag.

Rather than paper over that, the doc comments state it and point a caller who needs certainty at opening the file — noting that the answer can change between any check and the open regardless. A predicate cannot promise more than the mode bits it read, and a name like IsWritable is a footgun if the limitation isn't written down.

Consistency

Follows the existing conventions:

• paths expanded via ExpandPath first, like every other function
• empty path, missing path, and non-regular file all report false
• shared hasOwnerBits helper, so the two cannot drift
• doc.go gains a "Permission Predicates" section

Tests

Table-driven in the existing style, covering empty / readable / write-only / read-only / directory / missing for both, plus HOME and ~ expansion. One Example and one Benchmark.

gofmt, go vet and go test ./... clean.