docrot v1.5.0 — Scanner v6: reference-style images counted

v1.5.0 — Scanner v6: reference-style images are counted

Found by a scanner-vs-API parity probe: the paid API's Markdown extractor already consumed [label]: url definitions; the open-source scanner silently skipped ![alt][label] uses — under-counting references in repos that use them (false-negative class; the mirror image of the v1.4.0 comment find).

Fix: full, collapsed, and shorthand uses resolve against their definitions. Conservative guards: link-only definitions are not images; unresolved uses produce nothing; labels case-insensitive; code/comment strips apply as before. Dataset schema version 5 → 6.

Tests: 8 new (resolve ×3, link-only exclusion, unresolved, case-fold, code-block, comment); suite 74 → 82 green.

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Correction (2026-09-29, same day): this release was initially tagged on a commit where the new tests raised 8 NameErrors (unqualified extract_image_refs call; the red suite run was not heeded before shipping). Product code was unaffected and separately verified. The tag is repointed to include the test fix, the suite is verified 82/82 on the tagged commit, and the CHANGELOG carries the public correction bullet.