Add behavioural parity gate (closes #8); drop duplicate Python CLI

[pyrex41]

Summary

Adds the behavioural parity gate requested in #8, and consolidates the CLI on the Go binary by removing the duplicate Python implementation.

Byte-identical kernel.kl across hosts is necessary but not sufficient — the same KL can still execute differently per target (integer width, symbol interning, hash iteration order, memoisation). That gap is what bit shen-cas on shen-rust. This adds an opt-in gate that proves the slice computes the same thing on every target it ships to.

ratatoskr parity PROG OUTDIR

Shakes once, then builds + runs the slice on each stage-2 target twice (two separate boots) and diffs output against a reference (--reference, default lisp) or a committed golden (--expect FILE).

check	catches
vs-truth	cross-target divergence (the shen-cas case)
two-boot	cross-boot nondeterminism
two-pass	in-process boot-order nondeterminism (shen-cas#5)

two-pass is target-agnostic via a fixture convention: print two identical passes separated by a line that is exactly ===. Exit 0/1/3 = pass / fail / nothing checkable. --time is advisory.

Verified PASS across all 8 targets:

target   build  vs-truth  two-boot  two-pass
go       ok     ok        ok        ok        425ms
js       ok     ok        ok        ok        73ms
julia    ok     ok        ok        ok        5532ms
lisp     ok     ok        ok        ok        429ms
lua      ok     ok        ok        ok        23ms
rust     ok     ok        ok        ok        1317ms
scheme   ok     ok        ok        ok        445ms
swift    ok     ok        ok        ok        583ms
parity: PASS (8 target(s) checked)

Negative path (corrupt golden → FAIL exit 1 with first-diff) and skip-only path (exit 3) also verified.

Also

• Fixture: tests/parity.shen + tests/parity.expected (content-addressed memo — the hash/intern axis).
• Drop duplicate Python CLI: ratatoskr_cli.py, test_portable.py, .github/workflows/portability.yml removed; go.yml already covers the same helpers + embedded builders.json on the 3-OS matrix. Unit tests for the new pure helpers added in parity_test.go. The uvx/pip packaging (pybin/ + pyproject.toml) is kept.
• Docs: new docs/parity.md; README subcommand table + parity-gate section + cross-platform paragraph rewritten for the Go binary.

Closes #8

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