Show a more useful error when a Nikon High Efficiency NEF fails to load - #812

Reported as "NegPy won't open this NEF" on a Z 8 file. It is not a NEF
fault: the camera recorded it in High Efficiency (HE / HE*), which is
intoPIX TicoRAW under a licensed codec.

Nothing in the tags gives it away. The TIFF Compression reads 34713,
"Nikon NEF Compressed" -- the same value a lossless NEF carries -- so
libraw parses the file, reports the full 8280x5520 sensor and a valid
crop, and only fails when the payload is unpacked, with "Unsupported file
format or not RAW file". To someone whose other NEFs all work, that reads
as a corrupt file. Only the payload distinguishes them: the strip opens
with a plain-text intoPIX vendor marker, and the ratio is 0.357 against
~0.55-0.70 for lossless.

`unsupported_raw_reason` recognises that marker and returns wording that
says what to do -- re-shoot Lossless Compressed, or convert to DNG. The
loader stays lazy: the check runs only once a decode has already failed,
so a file that works never pays for it (measured 1.5 ms on a lossless
NEF, which is 1.5 ms saved on every decode of every good file). Anything
it does not recognise keeps libraw's own message, which is the honest
answer for a genuinely corrupt or unknown file.

Ruled out on the way: stripped MakerNotes (intact, 67 KB across 77 Nikon
tags) and a truncated file (every other IFD reads, including a
full-resolution JPEG preview).

Verified through the app headless -- the status line, the error signal
and load_failed all carry the new wording -- and the detector is tested
against a byte-built synthetic HE NEF as well as the real one, so it does
not depend on a single sample.