Report a malformed trace line as the exit-2 document, not a traceback

[Shashankss1205]

What

grapharc trace, grapharc metrics and grapharc viz crashed with a raw pydantic ValidationError traceback (exit 1, empty stdout even in --json mode) when the trace file contained a line that is not a valid TraceEvent — a truncated final line from a process killed mid-write, a hand edit, any stray line. The CLI's own contract in grapharc/cli/output.py names "an unreadable trace" as EXIT_UNAVAILABLE = 2, and exit 1 is reserved for "the command ran and the answer was negative" — so scripts drew the wrong conclusion, and replay/diff answered the same broken file politely while their three siblings crashed.

How

• grapharc/observe/trace.py: read_events now raises TraceReadError(path, line_number, cause) when a line does not parse, with a message naming the file and the 1-based line — unreadable trace file: /tmp/bad.jsonl: line 1 is not a trace event. It keeps raising rather than skipping: a partially-read audit trail presented as complete would be worse than a refusal. Exported from grapharc.observe alongside TraceRecorder.
• grapharc/cli/main.py: _cmd_trace, _cmd_metrics and _cmd_viz catch it and route through fail(...) with the default EXIT_UNAVAILABLE, following the _existing_trace / ReplayError-handler shape already in the file. Text mode: error: … on stderr, empty stdout. JSON mode: the failure is the one document on stdout, stderr empty.
• tests/test_cli.py: two parametrized tests over all three commands — text mode asserts exit 2 with the exact error: line naming the file and line number, JSON mode asserts exit 2 with exactly one {"ok": false, ...} document and empty stderr. Verified red with the source fix reverted (all six raise ValidationError on today's tree).

Out of scope, per the issue: replay/diff, skipping bad lines, the server's trace endpoints, and thread_summary's incremental index are untouched. Well-formed traces read byte-identically to before, so no README/cookbook transcript changes.

Verification

• pytest: 1647 passed, 12 deselected (including the byte-compared README/cookbook transcripts)
• ruff check .: clean
• Manual repro from the issue: all three commands now exit 2 with the report, no traceback

Fixes #15

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