feat: unless exception clauses on require/forbid/permit/expect

[rmichaelthomas]

Summary

Adds unless <exception> exception clauses to the four condition-grammar verbs — require, forbid, permit, expect — per Checkpoint v28 (Unless on Deontics Design Lock). Zero new reserved words: unless was already a connective via the existing when ... unless guard grammar.

Phases completed:

• Parser — exception: ASTNode | None field added to RequireNode/ForbidNode/PermitNode/ExpectNode (default compare=True, since the exception is semantic content, not inert metadata). unless consumed inside each of the four verb parsers via a shared _try_consume_unless_exception helper, positioned before because/inherited/temporal metadata. require each and when are untouched.
• Interpreter — exception-aware evaluation in _exec_require/_exec_forbid/_exec_permit/_exec_expect: require/forbid/expect are excused when the exception holds; permit is narrowed (its emission suppressed) when the exception holds.
• Renderer — unless <exception> renders between the condition and the because suffix, in both render and render_with_explicit_precedence.
• Analyzer — exception validated via _check_choose_condition; mixed and/or in the exception triggers the amber prompt (this logic actually lives in parser.py's _contains_mixed_precedence/_condition_is_mixed, not analyzer.py as the original spec assumed — corrected during implementation); detect_contradictions now carries guarded deontics through with conditional wording (... unless <exception>[ or <exception>]) instead of excluding them.
• Docs — new "Exception clauses (unless)" section in docs/language/syntax.md.
• Tests — 43 new tests across test_require.py, test_forbid.py, test_permit.py, test_expect.py, test_contradiction_detection.py, test_render_roundtrip.py.

Invariants verified:

• Vocabulary count unchanged — the public 60-word vocabulary (VERBS|CONNECTIVES|OPERATORS|ARTICLES|V2_RESERVED|MULTI_WORD_RESERVED|DECLARATIONS) is identical before and after; vocabulary.py was not touched. (Note: len(ALL_RESERVED) prints 61 both before and after this branch — a pre-existing, unrelated fact caused by the tombstoned combine word being included in that constant.)
• RequireEachNode and WhenNode are untouched.
• Render round-trip holds for all four verbs with unless, including full canonical metadata order.
• Backward compatibility: all pre-existing tests pass unmodified.

Test plan

• pytest tests/ — 1557 passed, 0 failed
• Smoke test: forbid total is above 10000 unless approved is equal to yes with approved = no → PROHIBITION_VIOLATED
• Smoke test: same with approved = yes → SUCCESS

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