runtime: implement RegExp sticky, dotAll, unicode, and hasIndices flags #2828

Summary

Perry's RegExp runtime records and implements only g, i, and m in the object header. Node exposes additional observable flags and matching semantics for y (sticky), s (dotAll), u (unicode), and d (hasIndices). Perry currently reports these flag properties as false and does not implement sticky matching or match indices.

Expected Node behavior

Local Node v25.9.0 probe:

const sticky = /a/y;
console.log(sticky.flags, sticky.sticky, sticky.global, sticky.lastIndex); // "y", true, false, 0
console.log(sticky.exec("ba"), sticky.lastIndex); // null, 0
sticky.lastIndex = 1;
console.log(sticky.exec("ba").index, sticky.lastIndex); // 1, 2

const dotAll = /a.b/s;
console.log(dotAll.flags, dotAll.dotAll, dotAll.test("a\nb")); // "s", true, true

const unicode = /./u;
console.log(unicode.flags, unicode.unicode); // "u", true

const indexed = /(a)/d;
const match = indexed.exec("a");
console.log(indexed.flags, indexed.hasIndices, match.indices[0], match.indices[1]);
// "d", true, [0, 1], [0, 1]

Current Perry implementation

Source evidence from current tree:

• crates/perry-runtime/src/regex.rs::RegExpHeader stores case_insensitive, global, multiline, and last_index, but has no sticky/dotAll/unicode/hasIndices booleans.
• js_regexp_new() derives only flags_str.contains('i'), contains('g'), and contains('m'); other flags remain only in the saved flags string.
• get_or_compile_regex() only injects Rust regex options for case-insensitive and multiline modes. There is no dotAll translation (s) such as (?s) / equivalent, so . will not match newlines for /s patterns.
• js_regexp_exec() treats only global as stateful: it starts at last_index only when global is true and resets/updates last_index only on that path. Sticky regexes should also be stateful and must match exactly at lastIndex instead of searching later in the string.
• crates/perry-runtime/src/object/field_get_set.rs routes RegExp property reads for .source, .flags, .lastIndex, .global, .ignoreCase, and .multiline, but returns false for .sticky, .unicode, .dotAll, and .hasIndices unconditionally.
• There is no visible support for the d flag's match.indices array in the js_regexp_exec() result path.

Suggested test surface

Add parity tests for the properties and stateful matching:

const sticky = /a/y;
assert.equal(sticky.flags, "y");
assert.equal(sticky.sticky, true);
assert.equal(sticky.global, false);
assert.equal(sticky.exec("ba"), null);
sticky.lastIndex = 1;
const sm = sticky.exec("ba")!;
assert.equal(sm.index, 1);
assert.equal(sticky.lastIndex, 2);

const dotAll = /a.b/s;
assert.equal(dotAll.dotAll, true);
assert.equal(dotAll.test("a\nb"), true);

const unicode = /./u;
assert.equal(unicode.unicode, true);

const indexed = /(a)/d;
const im = indexed.exec("a")! as any;
assert.equal(indexed.hasIndices, true);
assert.deepEqual(im.indices[0], [0, 1]);
assert.deepEqual(im.indices[1], [0, 1]);

Scope / non-goals

This is separate from earlier RegExp allocation/router fixes such as #637. It is specifically about RegExp flag property values and the matching/result semantics those flags enable.