Read references with the parser, not with str.split (#104)

[jeremymanning]

Follow-on from the triage in #457. The single largest remaining failure cluster — Undefined task reference across 23 files — was mostly the validator misreading its own input.

The defect

_extract_template_variables chopped raw template text: split('|')[0], then split(' ')[0]. That does not survive an expression, so the fragment was looked up as a task id and reported against the pipeline:

Undefined task reference: '(row'
Undefined task reference: 'from_json)'
Undefined task reference: 'analysis_topics[loop'
Undefined task reference: '(fact_check_loop'

None of those is a name anyone wrote.

Jinja already knows what a reference is, so it's asked instead — the same move #451 made for global calls. The AST also supplies what text-chopping could not: which names a {% for %} binds, and where a chain like a.b.c starts and stops.

This catches more, not less

A 14-file jump in the catalogue is exactly the shape of a validator going blind, so here's the check. split(' ')[0] on

{{ 'x' if ghost_step.done else 'y' }}

yielded 'x' — a literal, discarded — so a reference to a step that does not exist went entirely unchecked. It's an error now, and test_a_typo_inside_an_expression_is_now_caught pins it. Verified end to end through the CLI as well:

  - Undefined variable: 'ghost_step'
  - Undefined task reference: 'ghost_step'
Validation ✗ FAILED (STRICT)

Two smaller fixes in the same area

• loop.index was an undefined task. loop is Jinja's own variable and item is ours, but the allowlist compared the whole reference — so a bare loop passed while {{ loop.index }}, the way anyone actually writes it, did not. It compares the base name now.
• {{ range(3) }} was a task named range. Jinja's globals and the pipeline's are both excluded.

Measured

	before	after
catalogue validating	34 / 117	48 / 117
blocking suite	711 passed	725 passed, 0 failed

For context, the catalogue was at 9/117 when this line of work started.

Mutations — three, all killed

mutation	tests that failed
restore the text-chopping extractor	7
compare the whole reference to the loop allowlist again	2
stop excluding environment-provided names	2

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