feat(cpython): xz snapshot, preload urllib.request and parsers, fix doctest

[chaliy]

What changed

• The embedded CPython snapshot ships xz-compressed (artifacts/python.wasm.xz, 4.99 MB) instead of gzip (~6.7 MB). build.rs decodes it with lzma-rs, a pure-Rust build-only dependency. Runtime is unchanged.
• The freed room preloads urllib.request (with http.client and email), tomllib, configparser and xml.etree.ElementTree in the snapshot. Importing them now costs nothing per call.
• doctest works again: it imported pdb, which is not shipped. A small pdb stand-in now covers it, and breakpoint() prints a notice and continues.
• mailbox is no longer shipped.

Why

The CPython crate was 10.25 MB, 236 KB under the crates.io 10 MiB cap. That cap decided what could be preloaded, so common stdlib imports still cost 0.1-0.47 s on every call.

Before / After

	Before	After
bashkit-cpython-wasm .crate	10,249,728 B	8,642,486 B
import urllib.request (per call)	466 ms	~0 (call total 3.9 ms)
import configparser / tomllib / xml.etree.ElementTree	123 / 120 / 104 ms	~0
python_call/cpython/print	4.03 ms	4.14 ms
first call in a process	~20 ms	20.7 ms
doctest.testmod()	ModuleNotFoundError: No module named 'pdb'	TestResults(failed=0, attempted=1)

Build cost: decoding the snapshot takes ~2.5 s in a debug build script, measured with lzma-rs 0.3.

Risk

• Low
• Building bashkit-cpython-wasm now needs lzma-rs (plus crc and crc-catalog). These are exempted in cargo-vet; the licenses are MIT/Apache.
• Rebuilding the guest now needs xz on the build host.

Checklist

• Tests added or updated: doctest_works_and_breakpoint_continues; stdlib_is_bytecode_only now checks that mailbox and bdb are absent.
• Backward compatibility considered: no API change; the artifact rename is internal to the crate.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
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