Published @xterm/headless has a broken `module` field and no `exports` map

[yyq1025]

The @xterm/headless package on npm declares "module": "lib/xterm.mjs", but no lib/ directory exists in the published tarball — the ESM build actually ships at lib-headless/xterm-headless.mjs. The exports map is missing entirely. Verified on 6.0.0 (latest) and 6.1.0-beta.288/6.1.0-beta.289:

$ npm view @xterm/headless@6.0.0 module main
module = 'lib/xterm.mjs'
main = 'lib-headless/xterm-headless.js'

$ npm pack @xterm/headless@6.1.0-beta.289 && tar -tzf xterm-headless-*.tgz | grep '^package/lib/'
(no matches — lib/ does not exist)

The repo's committed headless/package.json has the correct module and exports values, but bin/package_headless.js regenerates that file at publish time by spreading the main package's package.json and only overriding name/description/main/types — so the main package's module (lib/xterm.mjs, valid only for @xterm/xterm) leaks into every @xterm/headless release and the committed exports map is dropped.

Impact: ESM-first resolvers that honor module try a nonexistent file. Bun silently falls back to main (CJS) so it happens to work there; stricter bundler configs can hard-fail, and nothing gets the intended exports resolution.

Fix is a few lines in bin/package_headless.js (override module and exports as well, matching the committed manifest) — PR incoming.

[yyq1025]

Prior art, for triage context: this was reported before as #5441 (v5.6.0-beta era) and fixed by #5442 — but that PR only edited the committed headless/package.json, which bin/package_headless.js regenerates (and clobbers) on every publish, so the fix never reached npm and the bug resurfaced in each release since. #5796 is the same issue re-reported against 6.0.0. That regeneration step is why #6053 patches the publish script instead of the manifest.