toLowerCase() does not apply the Greek Final_Sigma conditional casing rule (Σ at word end → σ instead of ς) #10116

What happened

String.prototype.toLowerCase() does not apply the Unicode Final_Sigma conditional casing rule: a Greek capital sigma (Σ, U+03A3) at the end of a word must lowercase to final sigma (ς, U+03C2), not medial sigma (σ, U+03C3). Node (and the spec, via SpecialCasing.txt's conditional Final_Sigma rule) gets this right; Perry always produces medial sigma.

node:  "ΟΔΥΣΣΕΥΣ".toLowerCase() === "οδυσσευς"   // wait — see below, corrected example
node:  "ΑΣ".toLowerCase()        === "ας"          // final ς
perry: "ΑΣ".toLowerCase()        === "ασ"          // WRONG: medial σ

Reproduced on current main with:

console.log("ΑΣ".toLowerCase());        // node: "ας", perry: "ασ"
console.log("ΟΔΥΣΣΕΥΣ".toLowerCase());  // node ends in ς, perry ends in σ

Root cause (source-read, not yet a fix)

case_convert in crates/perry-runtime/src/string/slice_ops.rs maps each decoded scalar via Rust's char::to_lowercase(), which only implements Unicode's unconditional simple/full lowercase mappings. The Final_Sigma rule in SpecialCasing.txt is conditional — it depends on the surrounding context (whether the sigma is preceded by a cased letter and not followed by one) — and Rust's char case-mapping API has no way to express that; it has to be implemented as a pass over the decoded scalar sequence with lookahead/lookbehind, not a per-character map.

Scope note

Not part of #10090 (the ASCII fast-path issue) — Greek text is non-ASCII, so it takes the same scalar case_convert path before and after that fix; this bug predates it and is unaffected by it. Discovered while writing #10090's regression test (test-files/test_gap_10090_string_case_ascii_fastpath.ts), which deliberately excludes final-sigma assertions with a comment pointing here.