Pair expected fixtures with canonical sources

[tmusser]

What changed

• added one canonical numbered *.source.txt file for each *.expected.json fixture
• standardized fixture anchors to Line N and Lines N-M
• made the schema validator require one-to-one source/expected pairs
• validate that every record anchor resolves inside the paired source
• reject anchored records with no substantive language overlap
• require follow-up actions to remain explicitly proposed
• documented the fixture-pair convention in examples/README.md

Why

The existing JSON fixtures were schema-valid and semantically plausible, but they were not paired with canonical source inputs. Anchors such as Thread or Alex line 5 could pass even when no validator proved that the referenced source existed or supported the record.

This change turns the fixtures from output-shape examples into grounded source/output evidence.

Pair contract

<name>.source.txt
<name>.expected.json

Source files contain sequentially numbered lines. Expected records must use Line N or Lines N-M, reference existing lines, and retain substantive overlap with those lines.

Validation

All checks passed on the final head:

• installer smoke tests
• repository structure validation
• example validation
• schema and fixture-pair validation
• numbered source parsing
• anchor format and range validation
• substantive source-overlap checks
• fixture-specific classification rules
• diff formatting