Replace the one-line `[file=... bytes=...]` render with a three-stage
pipeline in ck_diag — error bag -> aggregator -> reporter:

- bag: the structured Diagnostic (code, message, FileId, byte span) is
  untouched; every machine consumer (# trap / trap.md reconciliation,
  `ckc check --traps`, playground) keeps reading the structured layer.
- aggregator: each span becomes a mark on its source line; marks whose
  context windows (line +/- 2) overlap group into one shared window.
  Pure function, unit-tested on its own.
- reporter: per window — unchanged `error[EXXXX]: message` head lines,
  `--> path:line:col` (1-based, cols in Unicode scalars), a line-number
  gutter excerpt, and one caret line per mark keyed by its code
  (`note` for a secondary label). Two errors at one position stack
  their caret lines. Zero-length span = single caret; EOF span clamps
  to the last real line, one past its end; multi-line span underlines
  its first line + `(spans N lines)`.

Style: Plain (deterministic, escape-free — all goldens/tests) vs Bold
(ANSI SGR bold marks; driver picks it only when stderr is a tty and
NO_COLOR is unset). Path rule: relative to the CWD when the file lies
beneath it, else as resolved (driver `display_path`).

Renderer API is now `render_file(diags, file, src, style)` — rendering
needs source text, threaded from where each driver path already holds
the loaded file (never loaded by ck_diag). `line_col` moved from
ck_cktest to ck_diag (single source for the trap surfaces).

Consumers updated: ck_cktest section render (per page), driver check
(file + project), compile front-end/typecheck/dispatch/recursion/mock
paths (DI-dispatch diagnostics over test bodies now render against
their own file via FileId instead of a "test" label), two ck_resolve
unit tests. Playground/ck-highlight consume the structured layer only —
no change.

Goldens: 136 .cktest fixtures re-blessed mechanically
(`CK_BLESS=1 cargo test -p ck_driver --test conformance`); every
removed line is the old `bytes=` form, codes/spans unchanged (the
untrapped-diagnostics ratchet still reconciles at exactly 112).
Within a file, diagnostics now render in source order (inherent to
window grouping) — collection order previously leaked stage order.

Docs: docs/impl/08-diagnostics.md gains the DIAG1 format spec; the 11
docs/errors/E*.md examples re-rendered through the real pipeline.

Gates: cargo fmt --check, clippy --all-targets -D warnings, and
cargo test --workspace all exit 0; `ckc run tests/corpus/helloWorld`
still runs clean.

@@ -113,0 +424,4 @@
+/// column, the caret run, its key (code or `note`), and — for a multi-line
+/// span — the `(spans N lines)` extent. Bold wraps the carets and key under
+/// [`Style::Bold`].
+fn push_caret_line(out: &mut String, mark: &Mark, diags: &[Diagnostic], pad: &str, style: Style) {

@@ -147,0 +190,4 @@
+            .or_default()
+            .push(diag.clone());
+    }
+    let mut out = String::new();