init: export generated files and install the module in one call

[eunomie]

Summary

Make a new SDK module usable from a single init call. Today, creating a module
with e.g. dagger call go-sdk init requires two manual follow-ups before it can
be used:

dagger call go-sdk init   # writes main.* + dagger.json
dagger generate           # generate the module's SDK bindings
dagger install            # register the module in the workspace

This folds generate and install into the single Changeset that init
already returns, so one call leaves a ready-to-run module. The work lives in the
shared polyfill, so every SDK inherits it once bumped (the per-SDK init opts
in with two lines — see the companion go-sdk change).

What changed

• polyfill — two reusable fork primitives:
  • PolyfillWorkspaceFork.withGeneratedModule(modulePath) generates a module
    that exists only in the fork's staged state (not on the host yet) and
    overlays the generated files. Codegen can't run from inside a Dang module
    function, so it routes through the workspace-module-generate helper,
    extended with a --staged-dir-id mode that loads the staged directory by ID
    and runs GeneratedContextChangeset.
  • PolyfillWorkspaceFork.withInstalledModule(modulePath, name) registers the
    module in the workspace dagger.toml under [modules.<name>], mirroring
    dagger mod install. Re-installing the same name is a no-op.
• init-template — the SDK init skeleton wires both primitives (generate
  always; self-install skipped for a root-level module at path .).
• polyfill-e2e — checks for staged generate, install, and install
  idempotency.

No API changes: init keeps its signature.

Verification

• End-to-end in a scratch hello-dagger-style project: a single
  dagger call <sdk> init --name mybuild -y produced the module +
  dagger.gen.go/go.mod/internal/... + a dagger.toml entry, and
  dagger call mybuild ... worked immediately — no separate generate/install.
• Full dagger check -m .dagger/modules/polyfill-e2e (11 checks, incl. 3 new)
  passes on engine v1.0.0-beta.3.
• Helper go test / go vet pass.

Follow-ups

• Companion go-sdk change wires its init to these primitives.
• The other SDKs (python / typescript / java) need the same two-line init
  wiring once this polyfill is bumped.