Represent async effects and await sites in TypedProgram

Parent: #439
Depends on: #407, #440, and #441.

## Summary

Represent asynchronous effects and await sites in backend-neutral typed HIR.

## Scope

- Assign stable HIR identities to every await expression.
- Record the completed result type of each async function and the asynchronous type returned by a call.
- Require `await` to appear in an async context unless the language contract explicitly defines another context.
- Require the awaited operand to implement the agreed future/poll contract.
- Preserve generic substitutions, imported function identity, variant payload types, and source spans.
- Reject recursive type/effect cycles with diagnostics rather than backend panics.
- Reject unsupported async FFI and entry-point forms according to the semantics issue.

## Completion criteria

- [ ] `TypedProgram` answers whether a function is async and resolves each await operand/result type.
- [ ] Valid generic and imported async calls retain concrete types after monomorphization.
- [ ] Invalid await contexts and operand types report the relevant source span.
- [ ] No downstream consumer calls AST-local type inference for async nodes.
- [ ] Positive and negative frontend fixtures cover nesting, control flow, variants, pointers, and early returns.

Start by reading the parent issue #439 and dependencies #407, #440, and #441, then trace the existing TypedProgram representation. Define the work against the listed completion criteria: async functions and await results must resolve correctly, invalid contexts must report source spans, and frontend fixtures must cover the named cases without AST-local async inference downstream.

Written by the indexing model from the issue text.