test: run tests with node:test, keep Jest for old Node.js versions - #292

`main` runs the tests with `node:test` (#292); convert `expect` to
`assert` so the file runs under `node --test` and the Jest adapter.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ThsbPWvW5vnYWEaFs2Q22F

* feat: emit the source map scopes field

A bundler rewrites an imported binding into a namespace read, and a
source map only maps positions, so a debugger cannot resolve the name
the source used. The Scopes proposal answers that by naming, per scope,
the expression each variable evaluates to.

Add map({ scopes: true }). A source declares what its names read through
a new OriginalSource argument, the chunk stream carries it beside the
source content, and the field is built while the mappings are written
rather than decoded back out of them.

* perf: load the scopes encoder only when the option asks for it

Requiring it from the two map assembly helpers put the encoder and its
lookup table in the module graph of every caller, which the memory
benchmarks measured as a constant cost on sources that never emit the
field.

* test: cover the finished-map path of the scopes encoder

The streaming writer reaches none of it, so a map handed in from
elsewhere went untested: the early returns, a segment naming no source
or one the map lacks, a source resumed after another, a name the map
already carries, and a value too large for one base64 digit.

* test: reach the scopes helpers through the public export

The cases required the helper module directly, so the getter that
exposes it on `util` was never run.

* docs: document the scopes option and the bindings a source declares

Also choose the mapping callback once instead of testing for the scopes
writer per segment. Interleaved against main on one machine, that test
cost OriginalSource.map({ columns: true }) a reproducible 1-3%; with the
callback chosen up front the same measurement is neutral.

* chore: expose _scopeBindings without @Private and regenerate types

`tsc` emits `@private` as a TypeScript `private` member, which makes
classes from two installed copies incompatible; `main` dropped these tags
when it moved to generated types (#291). Regenerate `types/` for the
scopes changes.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ThsbPWvW5vnYWEaFs2Q22F

* test: port the scopes tests to node:test

`main` runs the tests with `node:test` (#292); convert `expect` to
`assert` so the file runs under `node --test` and the Jest adapter.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ThsbPWvW5vnYWEaFs2Q22F

* fix: close the last scope range where the generated code ends

The scopes writer closed the open range at column 0 of the last line.
When the output does not end with a newline, a range that starts on that
line began after that point, so its end encoded a negative column delta,
which the unsigned VLQ writer turned into 4294967292 and an invalid
`scopes` field. Pass the generated end column through `finish`.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01ThsbPWvW5vnYWEaFs2Q22F

---------

Co-authored-by: Claude <[email protected]>

`test/__mocks__/createMappingsSerializer.js` validated each mapping, but it
was never loaded, and it wrapped the serializer's default export, which the
library no longer uses: `getFromStreamChunks` and `streamAndGetSourceAndMap`
write through `createMappingsWriter`. #292 removed it as dead code.

`test/helpers/validate-mappings.js` brings the check back for the writer.
It replaces the module's `createMappingsWriter` export before any library
file loads, so every writer checks that mappings come in generated order
and that a segment without a source has no original position or name. A
stream that breaks that now fails the test that produced it instead of
being encoded into a quietly wrong `mappings` string. `node --test` loads
it with `--require`, Jest with `setupFiles`, so one file serves both.

The whole suite passes with the check. The one exception was a unit test
feeding the writer a synthetic stream whose source-less segments carried
original positions; those segments now use -1 like the library does.


Claude-Session: https://claude.ai/code/session_01ThsbPWvW5vnYWEaFs2Q22F

Co-authored-by: Claude <[email protected]>