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feat(concerto)!: native ESM migration for per-export tree-shaking (v5.0.0) (#1306)

* feat(concerto): add tree-shakeable package entrypoints

Signed-off-by: Rishabh Jain <rishabhj2005@email.com>

* fix(concerto-core): remove ESM entrypoint from sideEffects to enable package-level tree shaking

Listing `./dist/esm/index.mjs` in sideEffects prevented bundlers from
skipping concerto-core entirely when nothing from it is imported.

Instead, emit `dist/esm/dayjs-setup.js` as a separate file and reference
that in sideEffects — keeping the real side effect (dayjs.extend() calls)
declared while allowing the main ESM bundle to be tree-shaken at the
package boundary.

Verified: importing only concerto-util produces a 59 KB bundle;
adding concerto-core jumps to 1.46 MB, confirming the package is
correctly excluded when unused.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Matt Roberts <code@rbrts.uk>

* fix(concerto-core): emit dayjs-setup as .mjs and document build order

The ESM dayjs-setup output lived at dist/esm/dayjs-setup.js. Since the
package is not "type": "module", Node parses a .js file as CommonJS, so
an ESM file with that extension is ambiguous. Emit it as .mjs and point
the sideEffects entry at the .mjs path so the dayjs.extend() side effect
stays correctly declared.

Also document in DEVELOPERS.md that the monorepo must be built in
dependency order (npm run build), because each package's exports.import
now resolves to dist/esm/index.mjs.

Addresses review feedback on PR #1263.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Matt Roberts <code@rbrts.uk>

* fix(concerto): address Copilot review on ESM tree-shaking PR

- e2e: import Page as a type-only import (it is used only as a type),
  avoiding a runtime named-import mismatch under some TS transpile modes.
- build-esm: gate the createRequire shim banner to the Node-only build.
  Browser-targeted .mjs outputs no longer carry a Node-only
  `import... (continued)

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