chore: release 1.3.0

[leehack]

Releases 1.3.0: bumps pubspec.yaml to 1.3.0, renames ## Unreleased to ## 1.3.0 in CHANGELOG.md, and adds one README line for #36.

Contents: #34, which fixes #30, #31 and #33 (input marking through filters on captured output, tojson and join, and member access on undefined), plus #36. For #36, the README now says a function passed to render is trusted: | safe on a string it builds is unescaped, even when built from captured output. Checked by running the README example: <I><B>. The render-cost re-measurement is on #36 (comment): +22% / +20 µs per unmarked render, +38% / +34 µs with a function passed, on an idle GCE host. No CHANGELOG entry for the README line; the behavior is already in the first 1.3.0 bullet.

It's a minor release because it changes output that templates observe (escaping of input in filtered block output, tojson, join and string operations; undefined member access), and some templates that used to render now throw where llama.cpp throws (number subscript of none, e.g. Devstral Small 2507 and QwQ-32B). The changelog lists each case.

Gates at 854783c: format 0 changed, analyze --fatal-infos clean, VM 1178 pass (11 skipped), Chrome CI lane 1076 pass (11 skipped), pub publish --dry-run 0 warnings.

Closes #36

After merge, tag the merge commit v1.3.0 to publish.