perf(parse): cut allocations and validate UTF-8 in the scan

Per-statement work no longer allocates where it does not need to: parseKeyPath returns the first key component directly, absolute paths and key buffers sit on the parser, the definition maps are created on first write, arrays presize, and a string without escapes is copied out in one piece instead of built byte by byte. Repeated keys share one string across array-of-tables elements through a parser-local intern table whose lookup works on the bytes, so a repeated key costs no allocation.

UTF-8 validity is no longer a whole-input pass before the parser runs: the scan validates where it meets a multi-byte sequence, and a SyntaxError now names the line the invalid byte sits on. TestParseRejectsInvalidUTF8 covers nine positions; docs/API.md describes the new reporting.

Long document: 67 664 to 31 765 allocations per parse, about 51 to about 106 MB/s; representative document: 157 to 104 allocations.

interpres.go

@@ -24,7 +24,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"unicode/utf8"
 )
 
 // A SyntaxError describes a malformed TOML document, including the 1-based
@@ -143,9 +142,9 @@ func parseWithOptions(ctx context.Context, data []byte, opts parseOptions, wantD
 	if opts.maxInputSize > 0 && len(data) > opts.maxInputSize {
 		return nil, nil, fmt.Errorf("interpres: input is %d bytes, over the limit of %d", len(data), opts.maxInputSize)
 	}
-	if !utf8.Valid(data) {
-		return nil, nil, &SyntaxError{Line: 1, Msg: "input is not valid UTF-8"}
-	}
+	// UTF-8 validity is not checked in a pass of its own: the scanner
+	// validates the multi-byte sequences where it meets them, so an invalid
+	// byte is reported on its own line instead of always on line 1.
 	maxDepth := opts.maxDepth
 	if maxDepth <= 0 {
 		maxDepth = maxNestingDepth