feat(schema)!: per-module language namespace in can:// ids

[rahlk]

Closes #114. Spec: docs/design/specs/js-language-namespace.md (committed in this PR).

src/schema/ids.ts hardcoded const LANGUAGE = "typescript" and every code id descended from it,
so a JavaScript module emitted can://typescript/<app>/lib/foo.js/fn. The first can:// segment is
defined as a language namespace, and for .js modules it named the wrong language. Measured on
nodejs/node/lib: all 406 pure-JavaScript modules read can://typescript/….

can://typescript/mix                              :Application anchor, unchanged
can://typescript/mix/src/index.ts/greet           TypeScript module
can://javascript/mix/mocks/dpapi.js/defaultDpapi  JavaScript module
can://artifact/mix/package.json                   artifact layer, still neutral

moduleIdOf now takes the app name and derives the namespace from the file key:
.js/.jsx/.mjs/.cjs are javascript, everything else typescript. assignIds.ts is the only
caller, so the change is contained to one construction site.

BREAKING

JavaScript ids change. Neo4j MERGE keys on id, so re-projecting creates new nodes for JS
modules instead of updating existing ones — a store built by an earlier version needs a rebuild, and
saved Cypher or persisted references to JS node ids must be updated.

The analysis cache is unaffected: ids are stamped per run because they embed --app-name, and
the cached tree is id-free.

The accepted inconsistency

The :Application anchor keeps saying typescript while owning javascript children. Chosen
knowingly over the alternatives: a neutral anchor (can://app/<app>, mirroring the artifact layer)
is the more coherent end state but moves every id in every projection rather than only JS ones,
and two anchors would break the single-:Application invariant that carries analyzer identity
(#43). A mixed repository has no single language, so any single-anchor scheme names one language or
none. Moving to a neutral anchor stays open as a follow-up.

Verification

• 237 pass / 6 skip / 0 fail (+11 from test/ids.test.ts); typecheck, build, and gen:schema
  all clean.
• The change surfaced itself: two existing assertions in test/checker-guard.test.ts failed because
  they hardcoded can://typescript/… for a .js module. That fixture holds both a .ts and a
  .js module, so it now serves as the two-namespaces-in-one-run assertion.
• Edge cases pinned: .d.ts resolves to typescript (not read as a .ts suffix on a .d file),
  and the regex is anchored so a directory named vendor.js does not make its .ts children
  JavaScript.
• Neo4j projection over a mixed fixture emits all three namespaces — can://artifact/,
  can://javascript/, can://typescript/.