Source-map generation for compiled JS output (post-0.6.0)

[oubiwann]

Context

Lykn compiles .lykn source to .js output. Currently there is no
mapping between compiled JS line/column positions and original
lykn source positions. This means:

• Stack traces from compiled code reference JS lines, not lykn
  lines.
• IDE debuggers can't set breakpoints in lykn source.
• DD-49 Rule 7's "bridged error-message format"
  ("isValid (valid?): ...") exists specifically because the
  compiled name in stack traces doesn't connect back to the user's
  source name. With source maps, the parenthesized form could be
  dropped — runtime stack traces would resolve to lykn locations
  directly.

What needs to be designed

Generate .js.map files alongside .js files. Standard source-map
v3 format. The compilation pipeline already tracks span
information through SExpr::Span on the Rust side and equivalent
on the JS side — that span data is the input to source-map
generation.

Open design questions:

• Per-file or bundled source maps?
• Include surface forms (post-expansion) or only kernel forms?
• Inline (data: URL) or external file?
• Build-pipeline integration (lykn build, lykn build --dist)?

Downstream effects when source maps land

• DD-49 Rule 7 bridging format becomes redundant; can be simplified.
• IDE / LSP integration becomes feasible (separate, larger work).
• Debugging UX improves significantly.

Scope

Large. Substantial new infrastructure plus integration across both
compilers. Post-0.6.0 milestone candidate.

References

• DD-49 Rule 7: docs/design/05-active/0049-identifier-mapping-lykn-js.md
• M7 closing report fast-follow #8
• Phase 2 plan "Out of scope" section: "Multi-file source-mapping
  for compiled JS (debugger UX) — Phase 3+"