fix(lexer): interpret \xHH hex and \NNN octal escapes in interpolating strings

[tannevaled]

Problem

Interpolating string literals ("...", %Q{...}/%{...}, interpolating heredocs) did not interpret \xHH (hex byte), \NNN (octal byte), or \uHHHH/\u{...} (Unicode) escapes — they dropped the backslash and leaked the literal characters. The basic escapes (\n, \t, \0, …) already worked, so the escape interpreter existed but was incomplete.

source	before	MRI 4.0.5 / after
"\x41"	"x41"	"A"
"\101"	"101"	"A" (octal)
"\u{41}"	"u{41}"	"A"
"\n", "\0"	newline / NUL	unchanged (already worked)

Fix

lexer.scanStringSegment is the escape interpreter shared by every interpolating string form (%Q/%{} and interpolating heredocs re-lex their body as a double-quoted string, so they funnel through it). The missing cases were added there, matching MRI 4.0.5 byte-for-byte:

• \xHH — 1–2 hex digits, greedy (\x41→A, \xff→0xFF, \x4→0x04). A \x with no hex digit degrades to a literal x (MRI raises a SyntaxError; this entry point has no error channel).
• \NNN octal — 1–3 octal digits, greedy, masked to one byte (\101→A, \12→\n, \377→0xFF, \400→0x00).
• \uHHHH (exactly four hex digits) and \u{cp cp …} (whitespace-separated codepoints), each emitted as UTF-8; out-of-range / surrogate values fall back to U+FFFD.

Single-quoted strings, %q{}, and the regexp pass-through are deliberately untouched.

Tests / coverage

New lexer/string_escape_test.go covers hex/octal/unicode plus edge cases: max digits, single digit, greedy boundary, non-octal-digit boundary, no hex digit after \x/\u, octal overflow (\400), surrogate/out-of-range codepoints, empty/unterminated braces, and the %Q/%{}/interpolation-tail forms. Every case was verified against ruby -e on MRI 4.0.5.

• Module coverage stays at 100% (CI -coverpkg gate).
• gofmt / go vet clean, CGO_ENABLED=0 build green, -race tests pass.

Known divergence

\x / \u with no hex digit (and an out-of-range \u) are SyntaxErrors in MRI; since scanStringSegment has no error channel they degrade gracefully (literal fallback / U+FFFD) rather than failing the parse.

🤖 Generated with Claude Code