calc: LET and LAMBDA names ignore case in any script

Excel matches LET and LAMBDA names case-insensitively in every script,
so =LET(dług,100,DŁUG*2) is 200. The evaluator compared them ASCII-only
and returned #NAME?. The three lookups (name evaluation, ISOMITTED, a
LET variable shadowing a function name) now fold Unicode case.

docs/function-parity.md

@@ -1,6 +1,6 @@
 # Function parity: worksheet functions and formulas
 
-> **Last reviewed:** 2026-10-15 · **Last updated:** 2026-10-15 · **Change:** minor (MEDIAN and AGGREGATE median avoid intermediate overflow) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
+> **Last reviewed:** 2026-10-15 · **Last updated:** 2026-10-19 · **Change:** minor (LET and LAMBDA names ignore case in any script) · **Target:** Microsoft Excel (Microsoft 365), Excel for Mac 16.113.4
 
 Excel's worksheet functions, category by category, against GridCraft's function library. Part of
 [target-app-parity.md](target-app-parity.md); gaps feed [gaps.md](gaps.md).
@@ -114,6 +114,7 @@ workbooks; results checked in as data, not Excel files) and fixing what it finds
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-19 | minor | LET and LAMBDA names compare case-insensitively in any script, as in Excel: `=LET(dług,100,DŁUG*2)` is 200 (was #NAME?); also LET-bound lambdas called or passed by name (`żar(2)`, `MAP(…,Λ)`) |
 | 2026-10-15 | minor | MEDIAN and AGGREGATE median use an overflow-safe midpoint (#1579); regressions cover finite extremes, tiny values and dependent recalculation |
 | 2026-10-13 | minor | Clear reports changed stored cells sparsely, including those beyond the first million coordinates; dependent formulas and removed spills update, while clearing empty merges still unblocks spills (#1032) |
 | 2026-10-12 | minor | USDOLLAR added as a Compatibility function (not on Microsoft's list, so the counts don't change); MIRR with no positive flow is -1, COUNTIF's bare `"<="` / `">="` skip empty strings, INDIRECT with `a1` = FALSE gives `#REF!` for A1-style text (#738) |