A host that renders diff-so-fancy's output (e.g. lazygit) needs to map a
rendered diff row back to its patch-space identity -- file, line type, and
new/old line numbers -- to act on the line the user points at. diff-so-fancy
strips the +/- markers and conveys the side by color, so that identity cannot
be recovered from the painted text; the pager, which still has it at render
time, has to state it.

Gate the emission on the OSC1717_METADATA environment variable so output is
byte-for-byte unchanged outside such a host -- harmless in a raw terminal, less,
or tmux. The host advertises the protocol versions it understands and
diff-so-fancy emits the highest mutually-understood one (just V1 today). See the
protocol in diff-line-metadata-osc-spec.md; this is the third reference
implementation after delta and difftastic.

The emitter tracks its own old/new line counters, seeded from each hunk header,
and classifies every content line by its leading +/- indicator -- read before
the existing code strips that indicator and rewrites the line. sanitize_display
strips OSC sequences, so the record is prepended to the already-sanitized line
rather than embedded in it. The path prefers the new-file side, falling back to
the old side for a deletion (whose new side is /dev/null), which also recovers
the path for a noprefix deletion that never reaches $last_file_seen.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

…ations

Annotate the structural rows too, so a host can anchor file and hunk
navigation on the headers themselves and can see files that emit no
content records at all: every row of a ruled file-header block carries
the file's `f` record (spec §6.4), and the row announcing a hunk
carries that hunk's `h` record with the hunk's first new-file line --
straight from the freshly seeded counters, so it is the @@ new start,
not the first changed line the rendered header displays.

`f` never carries line numbers (the spec fixes each header type's
payload; a streaming renderer does not know the first hunk's line when
it draws the file header). That is what makes headers available for
files with no content lines, which now stay visible to the host:

- a pure rename or copy tags its ruled block,
- a binary file tags its "(binary)" block,
- a mode-only change tags its single announcement row -- gated on a
  lookahead so that a mode change on a file that also has content does
  not produce a second, differing header block (the ruled header that
  follows is that file's header),
- a combined/merge diff still has no annotated content (its multiple
  old sides do not fit the line-number model), but its file header now
  carries `f`, keeping conflicted files reachable.

With shortHeaders on, no hunk rows are rendered at all, so there is no
row for an `h` to tag; hosts fall back to content records.

With OSC1717 unset, output remains byte-for-byte identical.