Point configuration errors at the exact line and make colour optional

[bgrewell]

Modelled on CORE's manifest error rendering (grewelltech/core, mvp branch) — a located, colourised snippet that marks the line to change.

Option errors pointed at the wrong line

A typo in evaluatte: on line 11 marked line 6, the start of the test:

  > 6 |   - name: check the service
    7 |     node: local
    8 |     type: execute

DART already captured positions for each node/test/step block and for their type:/node: keys, but not for individual option keys. It now records those too, so the error marks the key itself:

     9 |     options:
    10 |       command: "true"
  > 11 |       evaluatte:
    12 |         exit_code: 0

ConfigError.Key — declared but unused until now — carries which option is at fault.

Syntax errors got no snippet at all

Error: yaml: line 2: did not find expected '-' indicator

The position was in the message text, and nothing rendered it. Any error carrying a YAML position is now converted to a located ConfigError on the way out of parsing:

Error: did not find expected '-' indicator

  suite.yaml

  ─────────────────────────────────────
    1 | suite: bad yaml
  > 2 | nodes:
    3 |   - name: local
  ─────────────────────────────────────

The conversion happens in one deferred choke point rather than at each yaml.Unmarshal — the first parse is inside substituteVars, so a per-call-site fix would have missed it. It is skipped once !!load_from has inlined other files, because the line numbers no longer match the file on disk and a snippet pointing at the wrong line is worse than none.

Colour is now optional

--color auto|always|never, defaulting to auto: colours when stdout is a terminal and honours NO_COLOR (the cross-tool convention). always forces it on for a pipe that renders ANSI; never turns it off.

It is resolved before anything can print, so an error rendered on the way out of parsing already honours the setting. An invalid value is rejected rather than silently treated as auto.

Verified: --color always emits escapes through a pipe, --color never and NO_COLOR=1 produce none.

What I did not take from CORE

CORE also separates a short Summary from a longer Detail and renders a column caret. DART's messages are already single-sentence and specific, and its SourceLocation.Column is populated but points at the YAML key rather than the offending token within it, so a caret would often be misleading. Left alone rather than added speculatively — happy to revisit if you want the two-part shape.

Verification

New tests cover position recovery from both YAML error shapes, that unlocated errors pass through untouched rather than acquiring a misleading location, that an already-located error keeps its own, and that an option key's recorded line differs from its block's. All example suites still pass --check; build, vet, gofmt, and the full suite are green; the docs site builds.