fix(build): stop publishing dangling source/declaration maps

[dmchaledev]

Summary

The published npm tarball ships 12 map files that all point at source that isn't in the package. package.json's files field publishes dist only (not src), but the build emits *.js.map and *.d.ts.map whose sources all reference ../src/*.ts. Since src/ is never published, every one of those maps is a dangling pointer.

Concretely, on the current main:

$ npm run build && node -p "JSON.parse(require('fs').readFileSync('dist/diff.d.ts.map','utf8')).sources"
[ '../src/diff.ts' ]          # <- not in the tarball

$ npm pack --dry-run          # 28 files, unpacked 49.5 kB
  dist/diff.js.map            # references ../src/diff.ts (absent)
  dist/diff.d.ts.map          # references ../src/diff.ts (absent)
  ... 12 map files total, 0 src files

Impact

• Declaration maps (.d.ts.map) exist for exactly one reason: IDE "Go to Definition" jumping from a consumer's editor into the original .ts. With src/ absent, that navigation resolves to a file that doesn't exist — the feature is shipped broken and misleading.
• Source maps (.js.map) break debugger source mapping the same way: stepping into the library resolves to non-existent ../src/*.ts.
• Dead weight: ~21 kB of the 49.5 kB unpacked size is maps that can never resolve.

Change

Disable sourceMap and declarationMap in tsconfig.build.json (the config used by npm run build → the published dist). declaration: true is kept — the .d.ts types are public API and must stay. Local dev/typecheck config (tsconfig.json) is untouched.

This makes the package internally consistent: it stops shipping maps it can't back with source.

Result

              main        this PR
files          28           16
map files      12            0
.d.ts files     6            6   (unchanged)
unpacked     49.5 kB      28.6 kB

Alternative considered

The other way to make the maps resolve is to add src to files and keep the maps, giving consumers go-to-definition into real TS. That's a legitimate product choice but bloats the tarball with raw sources; for a compiled library, dropping the maps is the standard, minimal-footprint fix. Happy to switch to the ship-src approach instead if source navigation into the package is desired.

Verification

• npm run typecheck — clean
• npm run lint — clean
• npm test — 29/29 pass
• npm run build + CLI smoke test — unchanged output; dist now contains .js + .d.ts only
• npm pack --dry-run — 0 map files, 6 .d.ts retained

Not a duplicate

None of the open issues (#9, #21, #22, #26, #27, #30) or PRs touch source maps, the files array, or published tarball integrity. This is confined to one build-config file and composes cleanly with everything in flight.

🤖 Generated with Claude Code

https://claude.ai/code/session_0121jPWdufr9tbgebBZAa5f9

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