fix(pyast): tag @modal.batched with a distinguishable leak (calque#91)

[scttfrdmn]

Summary

• Part of Lower-priority/rare Modal constructs: Dict/Queue, @modal.batched, NetworkFileSystem, CloudBucketMount, Cron/Period objects #91. @modal.batched(...) (Modal's auto-batching decorator) had zero recognition in tools/pyast/pyast.py — unlike its four sibling rare constructs (Dict/Queue/NetworkFileSystem.from_name(...), CloudBucketMount(...)), which already get a distinguishable leak tag via _unsupported_construct_from_name/_unsupported_construct_call. This one fell through completely unnoticed: no leak at all.
• _describe_fn now scans a function/method's decorator_list for a trailing "batched" name (mirrors the existing _decorator_name(d).rsplit(".", 1)[-1] == "function" style already used elsewhere) and appends a "modal.batched" leak to the same helper_leaks list the other four constructs use. It's a MODIFIER on top of whatever shape the function already has, so it deliberately does NOT become a new entry_kind value (unlike _SERVE_DECOS/_entry_kind) — the function still runs, just without Modal's request-coalescing behavior.
• No Go-side code changes needed: internal/parse/parse.go's existing generic HelperLeaks -> KindUnhandledCase channel picks up the new leak automatically (same mechanism Resolve real per-env images for a module-level for-loop of decorated functions (blocks #178 verification) #179's for-loop-expansion leak used with zero Go changes) — verified by actually running the new test.
• New minimal synthetic fixture testdata/scripts/batched_function.py (no vendoring, matches this repo's fixture conventions).
• Updated docs/modal-compatibility-matrix.md's @modal.batched/@app.batched rows from ⬜ "not modeled" to 🟨 "recognized, not modeled."
• Real batching execution (coalescing N concurrent calls into one list-valued call) remains explicitly out of scope — matches this project's "fake behind the seam, leak the gap" posture for rare (~2/212) constructs.

Test plan

• go build ./..., gofmt -l . (no output), go vet ./... all clean
• go test -count=1 ./... — all packages pass
• golangci-lint run ./... — 0 issues
• uvx ruff check . — all checks passed
• New TestParseModalBatchedDecoratorLeaks in internal/parse/parse_test.go, run verbosely (-run TestParseModalBatchedDecoratorLeaks -v) — PASS: asserts the leak's Detail mentions modal.batched + batching/coalescing, AND that the decorated function (process) still resolves normally via app.FindFunction (leak is additive, not a refusal to parse)