Remove the josh-starlark crate and document the wasm filter

[christian-schilling]

Remove the josh-starlark crate and document the wasm filter

Delete the josh-starlark workspace member, dead since the wasm filter op
replaced Op::Starlark. The starlark/allocative dependency stack leaves the
native Cargo.lock and survives only inside the excluded josh-guest workspace
(the wasm32 interpreter artifact).

Replace the Starlark section of docs/src/reference/experimental.md with the
wasm filter reference: ':!path=arg,...[context]' semantics, resource-limit
environment variables, .wat text support, the ABI v1 export/import tables,
josh-filter-guest authoring, and the Starlark-via-interpreter-blob recipe
(script API carried over; filter.starlark is now filter.wasm). Regenerate
docs/po/zh_CN.po with mdbook-xgettext/msgmerge and translate the new
entries; the merge also catches up catalog drift from earlier doc changes.
Update the upcoming-features bullet in README.md and workspaces/zh_CN.

Port the compose CI pipeline: ws/test.josh now carries static per-category
test nodes reproducing what the deleted ws/tests-per-category.star generated
for the current tests/ subdirectories (cli, cq, experimental incl. its
JOSH_EXPERIMENTAL_FEATURES=1 env insert, filter, proxy); the file stays
plain flang, parseable by the pinned bootstrap josh. The interpreter-blob
build is not wired into compose CI yet: the wasm_starlark_*.t tests skip
(scrut exit 80) when neither a wasm32 toolchain nor a prebuilt blob
(JOSH_STARLARK_GUEST_WASM) is available, keeping the experimental category
node green in containers.

The filtered-commit SHA goldens in wasm_{args,basic,chain,history}.t were
captured on a non-UTC host; update them to the values the containerized
test environment produces (the reference for all other suites). With that,
'JOSH_EXPERIMENTAL_FEATURES=1 josh compose run' is fully green.

Change: remove-josh-starlark
Assisted-By: anthropic/claude-fable-5