bundler: remove mutable Rootname global; derive and return the entry name (#95)

[argonui]

Summary

bundler.Rootname was a mutable package global that UnbundleAll rewrote at runtime, while findNextBundledScript hardcoded "__root" in its terminating regex. The two disagreed, producing the two failures described in #95:

• Silent module loss — a bundle whose entry module isn't __root lost its alphabetically-last module, with no error, because that module's __bundle_register block is only matched when followed by the literal return __bundle_require("__root").
• Cross-call contamination — the rewritten global persisted, so a later Bundle() registered the root under the stale name while metasuffix still emitted return __bundle_require("__root"), producing Lua that references an unregistered module.

Fix

Follows the issue's suggested approach:

• Rootname is now an immutable const — the canonical entry name and the default when a bundle declares none.
• UnbundleAll detects the actual entry name locally from the return __bundle_require("...") line, threads it into the scan regex (via regexp.QuoteMeta, no longer hardcoded), and returns it: (map[string]string, string, error).
• UnbundleAllXML returns the canonical Rootname so Lua and XML unbundling stay uniform behind the shared handler func pointer.
• Callers updated: Unbundle, AnalyzeBundle, handler.WhileWritingToFile (no longer references bundler.Rootname), and the affected tests.

Tests

Adds two bundler tests over a bundle with a foreign entry name (custom_entry):

• TestUnbundleForeignEntryName — asserts the alphabetically-last module (zzz) survives and the returned root name is correct (failure mode a).
• TestNoCrossCallContamination — unbundles the foreign bundle, then asserts a subsequent Bundle still emits __root and round-trips (failure mode b).

Verified the first test fails against the pre-fix hardcoded regex.

go build ./..., go vet ./..., and go test ./... all pass.

Fixes #95