feat: support PEP 798 unpacking comprehensions

[StudentWeis]

Python 3.15 permits unpacking in comprehension bodies, but tuple operands such as [*("prefix", x) for x in xs] and dictionary unpacking such as {**x for x in xs} currently produce ERROR nodes instead of comprehension trees. Allow list_splat bodies in list/set comprehensions and generator expressions, and dictionary_splat bodies in dictionary comprehensions.

Fixes #352.

The handwritten grammar change is four lines, following the focused approach linked in the PEP 798 discussion. It leaves the general list_splat_pattern rule unchanged (unlike the broader changes in #320).

Four corpus cases cover 14 expressions: identifier, tuple and call operands, dictionary unpacking, filters, multiple for clauses, async comprehensions, and a generator used as the sole call argument. The existing Lists expectations are also corrected: [*a.b], [*a[b].c], and [*a()] now place the complete expression inside list_splat, rather than treating the splat as the attribute receiver or callee (related to #314).

Generated files are included, regenerated with CLI 0.25.9 to match the repository. The large parser.c diff is generated parse-table churn; a second generation reproduces all three generated files byte-for-byte.

Validation on Linux x86_64:

• tree-sitter test: all 121 corpus cases pass; 34 highlighting assertions and 5 tag assertions pass.
• cargo test --locked: Rust binding test and doctest pass.
• npm run lint and git diff --check: pass.
• Compiled cccc's Python adapter against this grammar and Rust tree-sitter 0.27.0 in a temporary harness: all five original repro variants have no parse errors and retain their loop node (cognitive/cyclomatic scores 1/2 instead of 0/1).

Node/Python/Go/Swift binding tests and the external example repositories were not run locally.