fix: make field invariant CI pass

[rita-aga]

Supersedes #110. Preserves the field invariant and related-field cross-invariant work, fixes CI failures, and opens against staging instead of main.

Changes

• Field invariants ([[field_invariant]]): cross-field validation on single entities with when/require predicate trees (leaves: absent, equals, empty; combinators: any_of, all_of, not)
• Extended cross-invariant grammar: related(Target, fk).<FieldName> (in|not in) [...] for arbitrary field names (not just status)
• Server integration: pre_upsert_field_invariant_checks wired into the OData write pipeline, new FieldInvariant violation type

CI fixes (vs #110)

• Replaced all production .unwrap() calls flagged by Integrity & DST Patterns gate:
  • chars.next().unwrap() → let Some(first) = chars.next() else { return false } in is_identifier() / is_valid_field_identifier()
  • table.get("key").unwrap() → match table.get("key") in FieldPredicate::from_toml_value()
  • registry.read().unwrap() → match state.registry.read() in field constraint checks
• Fixed rustfmt drift across all changed files
• Split field_invariant.rs tests into separate file, extracted field_constraints.rs to keep files under readability ratchet limits
• Updated readability baseline for the net-new file count

Verification

• cargo build --release — clean
• cargo test --workspace — all pass
• cargo clippy --all-targets — zero warnings
• cargo fmt --check — clean
• cargo test -p temper-server --test field_invariants -- --nocapture — 6/6 pass
• cargo test -p temper-spec field_invariant — 21/21 pass
• cargo test -p temper-spec parse_field_assert — 6/6 pass
• Readability ratchet — all metrics pass
• Pre-push 4-gate pipeline — all gates passed

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