syntax: a results buffer is coloured as the code it quotes

+Grep, +Search and every language answer render rows like

    src/look.zig:718:12-16 fn grepText(path: []const u8, text: []const u8...

— a location, a space, and a piece of some file. The location names the file,
the file names the grammar, and the rest of the row is a fragment of that
language, so a grep over Zig reads as Zig and one Markdown row in the same
buffer does not pretend otherwise. The location itself is left uncoloured: it
is not code, and painting it as code is how a path starts looking like a
keyword.

`look.parsePathLine` decides what counts as a location — the same primitive
n/N already walks these buffers with, so the two agree by construction about
which rows are locations. NOT `lookableLineSpan`, which is n/N's whole
heuristic: it calls `resolve`, and a `realpath` per row per scroll is not
something a render path can afford. A bare filename is refused too; only
`path:line` counts, or a prose line whose first word ended in `.md` would
colour the rest of a sentence.

THE BUFFER IS COLOURED WHOLE, ONCE, WHEN IT IS FILLED. An adversarial pass
measured the obvious per-window implementation and it was untenable: the rows
are independent, so a window pass buys no fidelity, only amortisation, and
pays a burst on every scroll that outran the covered range. Grammars compile
their highlights query on first use — zig 26.9ms, cpp 18.9ms, rust 14.3ms —
so a polyglot grep showing six languages stalled a frame by 66ms, moving a
cost the syntax module had deliberately put on "opening a file" onto a scroll.
It also raised tree-sitter's allocation rate 3.5x (8,785 per refresh against
2,454) into a 16 MiB bump arena that only reclaims LIFO, so ~16 scroll
re-highlights exhausted it — and that arena is shared with real file panes, so
a results pane could evict editing. A grep is capped at 512 rows; colouring it
once makes the covered-range check true forever after and scrolling free.

The rest of that pass, in the same spirit: injections off for a single row
(both build a SECOND parser, per fenced block and per inline node, which is
absurd for one truncated row that almost never contains a fence), one query
cursor for the buffer instead of one per row, a one-entry extension memo so
non-matching rows stop paying a 29-spec scan, and NO highlights at all when
nothing painted — an all-zero run is not the same as none, and it defeated
`recolorSyntax`'s fast path, making every +Help and +Config walk its graphemes
every frame to paint nothing.

One correctness bug from the same pass: a failed `setLanguage` has already
nulled the parser's language, so leaving `held` on the previous grammar made
every later row of it skip the call and silently lose colour.

Documents keep `.source`: a New scratch and a real file are output-shaped but
have one language and an edit per keystroke, and `saves` is the line between
the two.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_016Q4RATpafkwahrovHQLKRf

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