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Add local-file ("folder") imports for py2js (§2+) (#379)

* Add local-file ("folder") imports for py2js (§2+)

Copies js-slang's own "folder" treatment for local, relative imports
(`from .foo import bar`), scoped to the py2js engine. Local file
resolution goes through IRunnerPlugin.requestFile — already published,
already backed by the frontend's folder-mode file store — so this needed
no conductor protocol change.

Implemented as a genuine source-to-source transform
(src/modules/localImports.ts, engine-agnostic): every locally-imported
file becomes a `def __module_x__(): ...` wrapping that file's own
statements, returning a real SICPy linked list of (name, value) pairs;
the importer's own `from .foo import bar` becomes a plain lookup against
it via a generated `__access_named_export__` walk — line for line the
same shape as js-slang's own local-import prelude. The whole multi-file
program flattens into one ordinary Python source string before anything
is parsed/compiled, so it's exactly the artifact a student can be shown
via `print(__program__)`, and needs no per-engine runtime support.

Requires SICPy §2 or higher, mirroring js-slang's own restriction to
Source §2+ for the identical reason: the pair/list transfer structure
doesn't exist below that chapter. CSE, PVML, and the WASM engine reject
a relative import explicitly rather than silently mistreating a dotted
path as a Source Academy module name.

Also fixes a real, separate bug this surfaced: Py2JsEvaluatorBase never
supplied `__program__` at all, so referencing it crashed every chapter
with a raw ReferenceError instead of failing gracefully (or working).
Py2JsSession now keeps `__program__` current itself, automatically, from
whatever text it actually compiles for a given chunk — mirroring the CSE
machine's own internal handling — rather than relying on an external
caller to remember to wire it up.

See source-academy/py-slang#378.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Clau... (continued)

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