fix: resolve ESM-only exports conditions when packaging (#281)

[robertsLando]

Closes #281

Problem

A packaged ESM app crashes at startup when it imports a dependency whose exports map is valid for ESM import but not resolvable through CommonJS require(). Plain Node runs fine; the packaged binary builds but crashes at runtime.

Two cases (from the repro):

• Import-only { "exports": { ".": { "import": "./index.mjs" } } } → ERR_PACKAGE_PATH_NOT_EXPORTED
• .mjs targets → MODULE_NOT_FOUND .../index.mjs

Root cause

pkg transforms ESM→CJS at build time and renames each dependency's .mjs files to .js in the snapshot, but left the dependency's package.json exports field untouched. Since Node's CJS resolver always prefers exports over main, at runtime require('<dep>') resolves against the original, unconverted exports map and fails. The pre-existing "synthetic main" workaround could never help, because Node ignores main whenever exports is present.

Fix

• New exported helper normalizeExportsForCJS() in lib/esm-transformer.ts (recursive): rewrites every .mjs string target to .js, and for any conditions object lacking a CJS-resolvable condition (require/node/node-addons/default) injects a synthetic require mirroring the import/default target.
• lib/walker.ts: rewrites the snapshot's exports field via normalizeExportsForCJS in the existing package.json patch block (alongside synthetic-main and type rewrites). Gated on !seaMode, matching the surrounding ESM-transform gating.

Tests

• test/unit/esm-transformer.test.ts — 7 unit cases for normalizeExportsForCJS (string, import-only, .mjs-target, default fallback, no-op when CJS condition present, nested/subpath recursion, arrays).
• test/test-54-esm-exports-conditions/ — e2e regression mirroring the repro (esm-only import-only package + req-mjs .mjs-target package). Fails without the fix (exit 1), passes with it (exit 0).

Verification

• Reproduced both crashes against the repro, confirmed correct output after the fix.
• tsc --noEmit, prettier, eslint all clean; existing ESM e2e tests (test-50/51/52/53) still pass.
• SEA mode intentionally excluded (no ESM→CJS transform there), consistent with adjacent logic.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.12%. Comparing base (546bbf0) to head (329d771).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/esm-transformer.ts	95.71%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #283      +/-   ##
==========================================
+ Coverage   86.36%   87.12%   +0.76%     
==========================================
  Files          22       22              
  Lines        7297     7389      +92     
  Branches     1047     1076      +29     
==========================================
+ Hits         6302     6438     +136     
+ Misses        988      943      -45     
- Partials        7        8       +1     

Files with missing lines	Coverage Δ
lib/walker.ts	87.77% <100.00%> (+0.50%)	⬆️
lib/esm-transformer.ts	89.08% <95.71%> (+1.06%)	⬆️

... and 2 files with indirect coverage changes

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