refactor(r13): method calls onto the registry; D3 case mode, P56, P57 (R13 slice 5)

[TimelordUK]

R13 slice 5: WHERE's hand-rolled method calls move onto the function registry. Every predicate operator and method call in WHERE now evaluates through the value evaluator — one implementation per method, in every clause.

Behaviour changes

• D3 (decided): Contains / StartsWith / EndsWith / IndexOf (method and function forms) and INSTR follow --case-insensitive like = and LIKE — case-sensitive by default. WHERE's .Contains() used to ignore case always; SELECT's never did, even under the switch. Queries relying on WHERE's old behaviour change answer; --case-insensitive restores it.
• P56 fixed: a method over a NULL receiver is NULL, not FALSE — WHERE NOT label.contains('e') no longer selects NULL labels (both evaluators).
• P57 fixed: any method may sit on the left of a WHERE comparison (WHERE s.ToUpper() = 'ABC' used to fail the query; only Length / IndexOf / Trim* were accepted).
• D2 extended: a numeric receiver is searched/trimmed as its displayed text (SELECT used to error on n.Contains('5'), n.Trim()).

How

• SqlFunction::evaluate_with_case (default ignores the mode) carries the switch into a function; MethodFunction::evaluate_method takes it; the value evaluator's three registry call sites pass it.
• One search_text reader in string_methods.rs for the search functions, LENGTH and TRIM*.
• WHERE delegates unconditionally; ~700 lines of its method arms, column resolver, literal readers, compare_trilean and did-you-mean suggester deleted.

Commits (pin → fix, R10 pattern)

1. Pin method calls — 3 matrices, DuckDB-derived; file P56, D3
2. Pin function forms
3. Registry search functions fixed (27 entries FIXED; 179 → 180 AGREE)
4. Pin methods WHERE rejected; file P57
5. WHERE delegates; arms deleted (28 entries FIXED; 180 → 183 AGREE)
6. Orphaned doc comment

Verification

• Parity: 179 → 183 AGREE / 213; each step moved exactly the predicted cases, contract holds.
• Rust integration 504 ✅, lib 834 ✅, Python 570 ✅, all 35 FORMAL examples ✅. Only failure is the expander_rewriters smoke example, which fails identically on main (unrelated GROUP BY error).
• A numeric Trim() receiver was missed by the pin and caught by the presidents FORMAL example; fixed and pinned.
• Three tests asserted the old always-case-insensitive rule; rewritten to assert D3 in both modes. No captured example output changed.
• Perf: Contains, Length() >, NOT StartsWith over trades_100000.csv all at load time (~2.5 s), level with LIKE.

Left for later

• Under D3: REPLACE, FREQUENCY, SUBSTRING_BEFORE/AFTER, SPLIT_PART, LEFT/RIGHT-with-delimiter don't follow the switch yet.
• LENGTH / INDEXOF / INSTR count bytes, not characters (unpinned).
• Next R13 step: slice 4 tail (AND / OR / NOT / CASE delegate), then slice 6.

Details: docs/ENGINE_REFACTORING.md § R13 slice 5; P56, P57, D2, D3 in docs/SQL_PARITY.md.

🤖 Generated with Claude Code