Make the library pack's bytes independent of the time zone

[alekswebnet]

Closes #51. Makes #48 unnecessary: close it unmerged, and no libraries release is needed.

What #48 is

The first weekly refresh since the pack was split rewrote all 106 archives, the 105 parts and libraries.h5p, and no library changed. Its change list is empty. Every archive was compared entry by entry between main and libraries/refresh:

• names, CRCs, sizes, methods, extra fields and comments: identical in all 106;
• the entry time: 2000-01-01 02:00 on main, 00:00 on the branch, in every entry.

Why

build-libraries.mjs and parts.mjs wrote every entry with new Date(Date.UTC(2000, 0, 1)). A zip records a wall-clock time with no zone, and zip.js takes it from the Date's local fields (getHours()). So the pack built at UTC+2 says 02:00 and the one CI builds in UTC says 00:00.

Changes

• FIXED_DATE in packages/libraries/scripts/parts.mjs is built from local fields, new Date(2000, 0, 1, 2), and exported. build-libraries.mjs uses it for the whole pack. It is 02:00 so that the bytes stay those pack 1.0.0 shipped, and the hashes player 1.0.x pins stay as they are.
• The workflow's change list matched only lines starting with H5P. A new FontAwesome, jQuery.ui, flowplayer, Shepherd or TimelineJS would have shown an empty list too. It now matches every library's line (105 of 105; the 6 header lines are left out).
• The workflow stops with an error when the files changed and no library's line in libraries.txt did, instead of opening a pull request that says the hub changed. That is either a build that is not deterministic, as here, or the hub changing a library's files without a new version; either way someone should look before a release.
• AGENTS.md: a sentence under the split pack's invariant.

Testing

• tests/unit/pack-bytes.test.ts (player, unit): builds a one-library pack the way build-libraries.mjs does and splits it with the real splitPack, in a child process per time zone (UTC, Europe/Kyiv, America/Los_Angeles, Pacific/Kiritimati). The bytes must be equal, and the first entry must record 2000-01-01 02:00. Against the old Date.UTC date both tests fail.
• The committed pack: node scripts/parts.mjs in UTC, Europe/Kyiv, America/Los_Angeles, Asia/Kolkata and Australia/Sydney reproduces all 105 parts and index.json byte for byte. TZ=UTC pnpm libraries, from the cached hub bundles, reproduces libraries.h5p byte for byte.
• Suites: the player's and the demo's typecheck pass; the player's unit tests pass, 343 of 343.

After merging

• Close Library pack 1.0.1 #48 unmerged. Its branch is pushed again by the next run that finds a real change.
• The next Monday run should find nothing to do.

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