Make check_source work on unbound rule templates

[rmichaelthomas]

Summary

check_source, as shipped in PR #65, calls run(source, enter_phase2=False, auto_confirm_amber=True) and passes contract_result.symbol_table straight to check_agreement. A contract source is very often an unbound rule template by design — it references evidence fields (amount, manager-approval, ...) that no remember binds. run() returns a symbol table with no entry for them, _build_constants allocates nothing, and encode_field raises UnencodableConstruct the first time a condition touches the field. Net effect: check_source returned encodable=False for every unbound template — every Translate draft, and every one of the six shipped templates in liminate-dev's app/agreement_templates.py.

This was verified against the real shipped vendor_invoice_review template, not just a synthetic case — it failed identically before this change. See Halverson dry run (PR #89), finding 1.

Changes

• _condition_leaves, _literal_type, _predicate_subject_types, _infer_field_types — a type-inference pre-pass that synthesizes type-only placeholder SymbolEntry objects (value=None) for referenced-but-unbound fields, inferred from a literal comparison or a defined predicate's own body.
• check_source now widens run()'s symbol table with these placeholders before calling check_agreement — a real remember/define binding always wins (only fills gaps, never overwrites).
• A field typed by neither path stays unresolved and still correctly reports encodable=False — invariant 8 unchanged.
• check_agreement, the encoder, and the seven checks are untouched.

Ported from case-studies/halverson/check-output/check_harness.py in liminate-dev — credit to that harness as the working prototype this stage productionizes.

Test plan

• check_source on an unbound template with a numeric threshold returns encodable=True and finds the expected violation (regression test for the whole stage)
• check_source on the real shipped vendor_invoice_review template (copied verbatim into the fixture) returns encodable=True
• A field typed by a real remember is not overwritten by inference
• A field typed only through a predicate application (<subject> is <defined-name>) resolves correctly
• A field referenced but untypeable still yields encodable=False with a skipped_reason, never raises
• Full suite green: pytest tests/ -q → 1787 passed (1782 prior + 5 new)

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