lsp: a protocol client for every other language, narrated on the message row

The seam grows a second backend: src/lsp/lsp_client.zig speaks JSON-RPC to
child language servers — rust-analyzer, clangd, gopls, tsserver, pyright are
rows in a spec table — while the in-process ZLS analyser keeps .zig. One
reader thread per server owns the socket, routes responses to a mailbox
under the conn mutex (monotonic condvar), answers server-to-client requests,
feeds the diagnostics store, and narrates $/progress and state changes
through a status sink both native shells post to the transient message row:
"rust-analyzer: cargo check 88% 955/1083" lands where a save narrates, with
the same clock. Chatty progress is throttled and deduplicated; settled
states always land, which is also what makes the goldens deterministic.

Nothing wedges and nothing healthy dies: waits are deadline-bounded, a
timeout cancels and returns no rows, three consecutive timeouts restart the
server ONLY while it is idle (an indexing server is narrating its own
excuse), spawn and handshake failures back off 10s to 2min, a crash shortly
after ready counts as a failure, and only a missing binary disables a spec.
PARDES_LSP_{RS,C,GO,TS,PY} override binaries; empty disables; the snapshot
harness pins RS to test/lspmock.zig and empties the rest.

Mutating answers really mutate now: the @put record beside rename @edit
carries per-range text, so = applies the formatter (both backends) and a
same-file WorkspaceEdit rename applies atomically, one undo step, narrated
("renamed 2 range(s)"); a multi-file rename previews as rows instead of
half-applying. Malformed responses fail closed: coordinates validated not
clamped, one bad TextEdit poisons the whole edit set, poison frames kill
the connection instead of buffering forever, decoded control bytes reject a
uri, hierarchy items too deep to reserialize are skipped.

Four kinds helix does not have, on SPC l: c/C incoming/outgoing calls (rows
are call sites), t/T super/subtypes. Pull diagnostics (3.17) preferred when
advertised. Help gains a language-keys footer for the motions no builtin
row could carry; lsp.rel and look.grep now share one path-shortening rule.

zig build lspprobe drives the seam from the CLI (comma-separated kinds share
one server); measured against a 1083-crate workspace warm: gd 26ms, gr 213
rows 165ms, incoming calls 212 sites 197ms, document symbols 670 rows 347ms.
docs/lsp.md tells the whole story; lsp-evaluation.md gets an addendum.

1 files changed, 16 insertions, 20 deletions

diff --git a/src/lsp/lsp_zls.zig b/src/lsp/lsp_zls.zig
index bf220c47..fb9804e6 100644
--- a/src/lsp/lsp_zls.zig
+++ b/src/lsp/lsp_zls.zig
@@ -1352,11 +1352,11 @@ fn codeActions(
 
 // ---------------------------------------------------------------- format
 
-/// `=`. The seam returns ROWS, not edits — there is no channel through which a
-/// backend can rewrite the buffer — so format reports rather than formats: one
-/// row per line `zig fmt` would change, in the `+Lsp` buffer. That is the
-/// honest shape of "format" behind a read-only seam, and it is still the
-/// answer to the question the keypress asks.
+/// `=`. The response is `@put` edit records (see lsp.put): one span covering
+/// everything `zig fmt` would change, which the core substitutes as a single
+/// undo step — so `=` FORMATS, exactly like helix. The two answers that are
+/// not edits stay prose rows in `+Lsp`: a file that does not parse cannot be
+/// formatted, and a file already formatted has nothing to apply.
 fn formatQuery(arena: std.mem.Allocator, req: lsp.Req, out: *std.Io.Writer) !void {
     const path = lsp.rel(req.root, req.path);
     var tree: Ast = try .parse(arena, req.source, .zig);
@@ -1377,22 +1377,18 @@ fn formatQuery(arena: std.mem.Allocator, req: lsp.Req, out: *std.Io.Writer) !voi
         lsp.row(out, path, 0, 0, "already formatted");
         return;
     }
-    var a = std.mem.splitScalar(u8, req.source, '\n');
-    var b = std.mem.splitScalar(u8, formatted, '\n');
-    var ln: usize = 0;
-    var n: usize = 0;
-    while (n < max_rows) : (ln += 1) {
-        const x = a.next();
-        const y = b.next();
-        if (x == null and y == null) break;
-        const xs = x orelse "";
-        const ys = y orelse "";
-        if (std.mem.eql(u8, xs, ys)) continue;
-        n += 1;
-        lsp.row(out, path, ln, 0, try std.fmt.allocPrint(arena, "- {s}   + {s}", .{
-            std.mem.trim(u8, xs, " \t"), std.mem.trim(u8, ys, " \t"),
-        }));
+    // one record, spanning only what changed: the common prefix and suffix
+    // stay untouched, which is also what keeps the cursor mapping tight
+    var lo: usize = 0;
+    const min_len = @min(req.source.len, formatted.len);
+    while (lo < min_len and req.source[lo] == formatted[lo]) lo += 1;
+    var src_hi = req.source.len;
+    var fmt_hi = formatted.len;
+    while (src_hi > lo and fmt_hi > lo and req.source[src_hi - 1] == formatted[fmt_hi - 1]) {
+        src_hi -= 1;
+        fmt_hi -= 1;
     }
+    lsp.put(out, lo, src_hi, formatted[lo..fmt_hi]);
 }
 
 // ------------------------------------------------------------------ files