Make effects a real, enforced, inferred type

[dalurness]

What & why

Effects are Clank's headline differentiator — but the checker didn't enforce them for builtins. This type-checked clean:

main : () -> <> () =   # declared PURE
  print("hello")       # ...but does I/O

Effect tracking was a syntactic perform-only pass (a W401 warning that run/check ignored), with no builtin effects, no cross-call propagation, and no higher-order effect polymorphism. The row-unification machinery existed but was dormant.

This moves effect tracking into type inference and makes an under-declared effect row a hard error (E401).

What changed

• Ambient effect inference. inferExpr threads an effect-scope stack: applications union consumed arrows' effect rows, perform records its op, lambdas accumulate into their own row (attached to the last arrow), and handle discharges handled ops in a child scope. The def-body row is checked against the declared row — subsumption preserved (declaring more than you perform is fine; open row-var tails absorb extras).
• Builtins carry real rows. io (print, fs writes, env, log, dt.now, cli, srv), io+exn (fs.read, http.*, proc.run/sh, csv file ops, streams; json.dec/as → exn), async (spawn, channels, sleep, ...), exn (raise).
• Effect polymorphism. map/filter/fold/flat-map share an effect var between callback and result — map : ([a], (a -> <e> b)) -> <e> [b] — so a callback's effect (inline lambda or named function) propagates to the caller.
• User functions carry their declared row (aliases resolved up front), so calls propagate effects transitively, including 0-arg calls.
• Effect rows render with <>; effect-row variables get readable a/b/c names. clank eval --type 'fs.read' → (Str -> <io, exn> Str).

Migration

10 test/*.clk files under-declared effects the checker now tracks (mostly adding exn to io mains that read files / decode JSON). Fixed. The cross-module effect-alias test moved to the propagation pattern, since discharging an imported effect's operation needs the effect declaration linked (ops aren't pub-exported).

Tests

12 checker tests (builtin io/exn/async enforcement, transitive + higher-order propagation incl. named callbacks, handle discharge, subsumption, pure pipelines stay pure) + 2 eval --type CLI tests. Full suite green.

Known follow-ups

• clank doc builtin signatures don't yet show effect rows (the doc registry is a separate approximate type table); clank eval --type is authoritative.
• Effects don't propagate through TAny-typed combinators (iter.*, srv.* handlers, spawn's thunk) — they accept effectful callbacks but don't propagate. Inline lambdas and the typed list HOFs do propagate.

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