Highlight the contextual plots keyword in figure and layer fields

[shunichironomura]

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Summary

Companion to graphcal-lang/tree-sitter-graphcal#80.

• Copy the query change: (figure_plots_field "plots" @keyword) and (layer_plots_field "plots" @keyword), so plots is highlighted like the other contextual keywords (fn, bind, plugin, mark, encode). languages/graphcal/highlights.scm stays identical to the tree-sitter repo's queries/highlights.scm.
• Pin the grammar in extension.toml to d812f2564fbf7c1cb2e6b3d1ac9aeb06566d5f82, the head commit of the tree-sitter PR branch.

Merge graphcal-lang/tree-sitter-graphcal#80 first. If that PR is squash-merged, re-pin rev to the resulting commit on main before merging this PR, since the branch commit may become unreachable.

Validation

• cargo check --locked and cargo fmt --check pass.
• The same query, run with tree-sitter query against the pinned grammar revision on a sample figure/layer with plots: [...], captures both plots tokens as @keyword.

AI assistance

• AI used: yes — query copy, grammar pin, and validation.
• Human review of AI-assisted code: pending.

Breaking changes

• No breaking changes
• Breaking changes; the breaking change label is applied