Make the README's snippets compile-checked Example functions.

[xbmlz]

Follow-on to #73, which fixed the README by hand and showed why hand is not
enough.

The README's Go snippets have no compiler behind them. That is exactly how the
write-diagnostic example in #73 came to be written on SetFloat, which rejects a
float for an IS at the call site and so could never reach the hook it was
demonstrating — I only caught it because I ran it. These four Example
functions cover the same ground and go stale loudly instead of quietly: go test compiles them and checks their output, and pkg.go.dev renders them beside
the API they use.

Example	README snippet
Example	set a value, encode, read it back
ExampleReadOptions	the read diagnostic hook
ExampleWriteOptions	the write diagnostic hook
ExampleDataset_Encode	dataset bytes with no File Meta

No fixture, no filesystem

An Example cannot take a *testing.T, so it has no t.TempDir, and a snippet
that opens a path the reader does not have teaches less than one that builds its
own bytes. Each example encodes a Part 10 dataset in memory and reads it back.
That is also what lets ExampleReadOptions cut four bytes off the end and
provoke a real truncated_value rather than describe one:

truncated_value at (0010,0010): need 8, have 4
PatientName present: false

partTen sets SOP Class and Instance UID because PS3.10 requires File Meta to
name them — EnforceFileFormat fills the File Meta from the dataset and refuses
without them. Worth a reader seeing once, so it is a commented helper rather
than hidden.

And it found one more README error

The write example's comment showed the diagnostic's own message, but a caller
gets it wrapped:

godicom: error writing dataset: godicom: invalid_value at (0018,0086) IS: ...

The README comment now says what err actually prints, and ExampleWriteOptions
pins it.

Gates run locally before pushing: gofmt -l (via the CRLF-safe copy), go build ./..., go vet ./..., go test ./..., and staticcheck -checks=all —
all clean. No non-test code changes; example_test.go is new, plus the README
comment fix and a Docs: changelog entry under Unreleased.