parse: support parenthesized parallel <*> in pkg/script (grammar parity)

[vinodhalaharvi-claude]

What & why

<*> parallel fan-out works in the original internal/agentscript grammar and always has. The newer pkg/script parser — the path loom uses — was built >=>-only (an MVP omission documented in the code), so parallel pipelines that ran fine in the CLI failed through loom. This restores parity: the same DSL that parses in the original runtime now parses in pkg/script. No capability the project already had should be missing from the unified pipeline.

Parser changes (pkg/script/parse.go)

• Add a Fanout token (<*>) to the lexer
• Grammar: pipeline = stage ( >=> stage )*; stage = call | group; group = ( pipeline ( <*> pipeline )* ). One-branch group = grouping; two+ branches = parallel fan-out
• toAST: new stageToAST/groupToAST emit ast.Parallel for multi-branch groups
• Invariant preserved: a block body is still always an ast.Pipeline; Parallel appears only as a stage

⚠️ An earlier draft collapsed single-stage pipelines and broke three existing parse tests. I caught it, reverted the collapse, and kept the always-Pipeline contract that Lower and the memory bridge depend on. No regression ships.

Downstream was already ready

ast.Parallel / resolved.Parallel exist, Resolve has the ast.Parallel case, and the memory bridge maps resolved.Parallel → the runtime's *Parallel. This PR is purely the parser catching up.

Verified end-to-end (new pipeline)

memory static ( ( ssl_check "google.com" <*> dns_lookup "github.com" ) )
→ parse → resolve → RunMemory → BOTH branches execute in parallel ✓

Complex grammar parses + resolves:

( search >=> analyze <*> search >=> analyze ) >=> merge >=> ask "..."

Tests

parse_parallel_test.go: parallel forms parse; multi-branch group → Parallel stage; single group is not parallel; complex grammar parse+resolves. All prior parse tests still pass (regression guard).

Step	Result
vet / gofmt / staticcheck	✅
go test -race ./pkg/...	✅
build	✅