build: a vendor bump rebuilds libghostty-vt, everywhere it is consumed

Three stale-build landmines, all found by shipping v0.1.7 -- the first
release where new code (GhosttySearch) actually needed the vendor bump
of 2026-09-01:

- $(VT_LIB) had no prerequisites, so an existing .a *was* the build:
  the macOS builder compiled pane.c against zig-out headers from before
  the bump. The vendor manifest is now the prerequisite; a re-vendor
  rewrites it, so a bump rebuilds the lib and its installed headers.

- web/demo had the same guard and also never passed -Dversion-string:
  upstream's build.zig ran git describe in our repo, and on the tag
  checkout web/publish builds from, a slosh tag is a fatal version
  mismatch. Both fixed; both had hidden behind a stale build/vt.

- The builder's zig-pkg (excluded from the sync, and left half-empty by
  a once-interrupted push) is an input, not a cache: it is now synced
  separately and additively, so a bump's new packages arrive and a
  partial local tree cannot maim the builder's copy.

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